WORKS  OF   E.   B.   WILSON,  E.M., 


PUBLISHED   BY 


JOHN   WILEY   &   SONS. 


Cyanide  Processes. 

lamo,  cloth,  $1.50. 

The  Chlorination  Process. 

i2mo,  cloth,  $1.50. 

Practical  Mine  Ventilation. 

For  the  Use  of  Mining  Engineers,  Students,  and 
Practical  Men.    With  plates.     i6mo,  cloth. 


-; 


THE 


CHLORINATION  PROCESS 


E.   B.   WILSON,   E.M. 


FIRST   EDITION'. 
FIRST    THOUSAND. 


NEW  YORK: 

JOHN   WILEY   &   SONS. 

LONDON:    CHAPMAN   &   HALL,   LIMITED. 

1897. 


Copyright,  1897, 

BY 

E.  B.  WILSON. 


76  J 
7 


ROBERT  DRUMMOND,    ELBCTROTYPBR   AND   PRINTER,   NEW  YORK. 


INTRODUCTION. 


THE  treatment  of  refractory  gold-bearing  ores  is 
of  interest  to  those  who  are  or  have  been  engaged 
in  mining  the  precious  metal. 

Some  wise  person  has  stated  "  that  all  is  not  gold 
that  glitters  ";  and  if  he  were  alive  and  a  miner  he 
could  have  added  two  other  facts  which  history  has 
established,  viz. : 

That  all  gold-bearing  rocks  do  not  contain  gold  in 
paying  quantities;  also,  that  some  gold-bearing  rocks 
contain  considerable  quantities  of  gold,  but  are  com- 
mercially valueless. 

The  greater  part  of  the  money  sunk  in  gold-min- 
ing ventures  has  been  due  to  the  above  facts;  but 
should  the  statement  be  discredited,  inquiry  of  those 
who  have  lost  money  in  the  past  will  substantiate  it, 
we  believe. 

The  chlorination  method  of  treating  refractory 
ores  has  been  some  years  before  the  public,  and 
while  old  yet  it  is  new. 

It  has  added  much  to  the  world's  store  of  gold, 


404228 


IV  INTROD  UCTION. 

and  is  destined  to  add  more  as  it  becomes  generally 
practised,  as  it  has  assumed  a  place  in  metallurgy 
from  which  it  cannot  be  dislodged. 

The  process  is  not  one  of  great  difficulty,  although 
it  has  been  belittled  by  those  who,  claiming  to  be 
mining  experts,  were  unable  to  practise  it,  and  who 
therefore  proposed  either  a  less  economical  plan  of 
treatment,  or  brought  financial  loss  upon  those  who 
followed  their  advice.  The  writer  takes  occasion 
here  to  express  his  obligations  to  those  whose  data 
he  has  included  in  this  volume,  and  trusts  he  has 
given  them  proper  credit. 

The  author  recognizes  that  owners  of  mines  as  well 
as  students  and  those  practically  engaged  are  inter- 
ested in  this  subject.  He  has  endeavored  for  that 
reason  to  place  the  subject  in  such  a  form  that  any 
one  who  reads  can  understand.  The  chemical 
formulae  are  of  little  value  except  to  those  who  are 
in  practice  or  intend  to  practise;  their  omission 
would,  however,  detract  considerably  from  the  value 
of  the  book  for  reference  and  completion  of  the 
subject. 

The  author  trusts  his  endeavors  will  meet  with  the 
approval  of  those  interested. 

E.  B.  WILSON. 

JUNE,  1897. 


CONTENTS. 


CHAPTER 

I.  Lixiviation  by  Chlorine  Solutions l 

II.   Preparation  of  the  Ore Il 

III.  Roasting  the  Ore 2O 

IV.  Roasting  furnaces 3& 

V.  The  Leaching  Process 6o 

VI.  Filtering 75 

VII.   Precipitation 8l 

VIII.  Refining  Precipitates 92 

IX.  Resume  of  Chlorination  and  Plant Q8 

X.  Cost  of  Chlorination IO5 

v 


THE  CHLORINATION   PROCESS. 


CHAPTER    I. 
LIXIVIATION   BY   CHLORINE  SOLUTIONS. 

LIXIVIATION  is  the  term  applied  to  the  abstraction 
by  a  liquid  of  the  soluble  part  of  a  mineral,  or  aggre- 
gation of  minerals.  An  aggregation  of  minerals  is  a 
rock. 

Lixiviation  is  therefore  the  abstraction  by  a  liquid 
of  the  soluble  part  of  a  rock. 

The  useful  processes  for  separating  gold  from  ores 
by  lixiviation  are  limited  chemically  by  the  few 
known  solvents  for  it,  and  commercially  by  the  cost 
required  to  effect  profit.  We  are  therefore  at  present 
confined  to  the  following  processes: 

Lixiviation  by  hyposulphite  soda  solutions,  or  the 
Russell  Process,  for  silver  first; 

Cyanogen  solutions; 

Bromide  solutions; 

Bromide-cyanogen  solutions; 

Chlorine  solutions. 


TION  PKOCESS. 


They  are  all  based  upon  the  solubility  of  gold  in 
aqueous  solutions  of  the  chemicals  named.  Lixivia- 
tion  by  chlorine  solutions  is  what  we  are  particularly 
to  deal  with.  Rose,  in  his  "  Metallurgy  of  Gold," 
gives  as  the  relative  dissolving  powers  of  the  last 
three  chemicals  in  i-per-cent  solutions,  with  each 
solution  at  the  temperature  of  60°  Centigrade,  the 
following: 

Chlorine  in  I  J  hours  dissolves          4.49$  of  gold. 
Bromine  "   ij       "  "  6.46$  "     " 

Cyanide   "   ij       "  "  o.; 


All  these  wet  processes  require  that  the  ore  be 
subjected  to  preliminary  treatment  before  using  the 
solutions.  With  chlorine  solutions  preliminary  treat- 
ment is  elaborated  more  than  in  the  bromine  or 
cyanide  solutions. 

All  four  processes  seem  to  occupy  a  distinctive 
field  of  their  own,  but  also  encroach  upon  each  other's 
territory.  In  such  cases  the  choice  of  the  process 
should  depend  upon  which  will  extract  the  greatest 
percentage  of  gold  at  the  least  cost. 

Could  we  say  that  any  one  of  the  processes  was 
the  best,  this  choice  would  be  an  easy  matter;  but  as 
each  ore  differs  in  character,  it  is  not  possible  to  say 
that  any  one  of  the  above  processes  is  the  best  under 


LIXIVIATION  BY   CHLORINE   SOLUTIONS.          3 

all  circumstances;  and  then  the  choice  of  process 
becomes  difficult,  and  can  only  be  decided  by  the 
metallurgist  after  experiment. 

Chlorination  (by  which  we  mean  the  leaching  of 
gold  ores  by  chlorine  solutions)  does  not  save  the 
silver  content  of  the  ore,  because  during  preliminary 
treatment  the  ore  is  subjected  to  chloridizing  roast- 
ing, which  forms  an  insoluble  compound  in  water, 
known  as  Silver  Chloride. 

The  reverse  is  the  case  with  gold,  which  forms  in 
chloridizing  roasting  a  gold  chloride  soluble  in  water. 
But  other  metals  may  do  the  same;  consequently  it 
becomes  necessary  to  remove  the  other  metals  by 
oxidation,  or  by  oxidizing  roasting. 

Under  certain  conditions  gold  unites  with  chlorine, 
forming  the  gold  chloride  known  as  trichloride  of 
gold,  or  auric  chloride,  where  one  triad  atom  of  gold 
unites  with  three  monad  atoms  of  chlorine,  forming  a 
molecule  of  auric  chloride  whose  molecular  weight  is 
302.31,  and  whose  chemical  symbol  is  AuCls.  The 
chief  source  of  chlorine  is  common  salt  (NaCl),  which 
contains  about  60  per  cent  of  this  substance.  As  a 
gas  it  has  a  greenish  yellow  color  and  a  very  dis- 
agreeable odor,  producing  on  inhalation  a  suffocating 
cough.  (The  cough  may  be  relieved  by  breathing 
ammonia  or  ether.) 

Cold  water  absorbs   about  twice   its  volume    of 


4  THE   CHLORINATION  PROCESS. 

chlorine  gas,  being  converted  gradually  into  hydro- 
chloric acid  (HC1)  by  the  chlorine  uniting  with  the 
hydrogen  of  the  water. 

Slaked  lime,  or  calcic  hydrate  (CaO  -f-  HaO),  when 
exposed  to  chlorine  gas  forms  a  chloride  of  calcium 
and  hypochlorite  of  calcium,  or  what  is  known  as 
bleaching-powder,  with  formula  CaCl2  +  CaCl3O2. 

Commercial  bleaching-powder  contains  from  20  to 
35  per  cent  of  available  chlorine.  It  forms  a  homo- 
geneous white  powder,  possessing  a  smell  of  hypo- 
chlorous  acid,  gradually  becomes  moist  on  expos- 
ure to  the  air  and  decomposes  with  absorption  of 
water  and  carbonic  acid.  It  should  therefore  be 
kept  away  from  the  atmosphere.  Bleaching-powder 
derives  its  chief  value  from  the  hypochlorite  of  lime 
which  it  contains. 

Hypochlorous  acid  is  so  weak  an  acid  that  its  salts 
are  easily  decomposed.  Carbonic  acid  gas  decom- 
poses it  readily.  The  salts  of  hypochlorous  acid  are 
unstable  compounds,  the  same  as  the  acid;  and  the 
calcium  hypochlorite  gives  the  bleaching-powder  its 
chief  value  both  for  bleaching  and  the  chlorination 
process,  from  the  fact  it  yields  its  chlorine  readily. 
When  either  hydrochloric  or  sulphuric  acid  are  added 
to  bleaching-powder  a  quantity  of  chlorine  equal  to 
the  quantity  in  the  hypochlorite  is  evolved.  The 
reaction  is  as  follows: 


LIXIVIATION  BY  CHLORINE  SOLUTIONS.          5 

( 2HC1  +  CaCLA  =  2HOC1  +  CaCla; 
'  (2HC1  +  2HOC1  =  2HaO  +  2Cla. 

n   (CaCla+CaClA+H3S04=2CaS04+2HCl  +  Cl9O9; 
( 2HC1  +  Cl20a  =  2H.O  +  2Cla. 

In  the  first  case  half  the  chlorine  is  obtained  from 
hypochlorite  and  half  from  the  hydrochloric  acid. 

In  the  second  case  the  sulphuric  acid  decomposes 
the  chloride  and  hypochlorite,  liberating  all  the  chlo- 
rine in  both  compounds. 

It  may  be  possible  to  obtain  liquid  chlorine  in  a 
concentrated  state,  thus  reducing  the  bulk  compared 
with  bleaching-powder.  To  dissolve  gold  by  chlo- 
rine, the  latter  must  be  in  a  free  or  nascent  state,  that 
is,  as  a  gas,  or  in  a  liquid  state,  uncombined  with 
other  chemicals,  as  chlorine  water.  The  gold  must  be 
in  a  metallic  state,  and  the  chlorine  will  then  combine 
with  it  if  oxygen  be  present.  Whenever  metallic 
gold  is  dissolved  in  nitro-muriatic  acid  (HNO8  +  HC1) 
chlorine  is  liberated  in  the  presence  of  oxygen,  and 
forms  with  gold  the  compound  HAuCl4  -j-  2HaO, 
known  as  chlo-auric  acid,  a  very  peculiar  combina- 
tion, the  reaction  of  which  is  as  follows: 

4HC1  +  CaClaOa  +  Au  +  HNO3  = 

HAuCl4  +  CaCla  +  N09  +  2H2O. 

The  deep  yellow  solution  obtained  gives  upon 
evaporation  yellow  crystals  of  the  double  chloride  of 


O  THE   CHLORINATION  PROCESS. 

gold  and  hydrogen  (HAuCl4),  and  this  cautiously 
heated  solidifies  to  a  red  crystalline  mass,  soluble  in 
water,  alcohol,  or  ether,  and  is  auric  chloride,  AuCl,. 
The  dissolving  of  gold  from  ores  by  the  above  proc- 
ess would  not  be  feasible  on  account  of  dissolving 
other  impurities  as  well,  together  with  the  subsequent 
difficulties  and  expense  in  separating  the  gold  from 
the  base  impurities,  and  it  is  merely  mentioned  as  an 
example  of  the  active  principle  of  chlorine  in  its 
attack  upon  gold  when  it  can  be  liberated  from  its 
compounds. 

It  can  be  liberated  from  its  compounds  as  readily 
as  in  the  above  example.  The  practical  results,  how- 
ever, depend  upon  its  liberation  in  contact  with  gold, 
in  aqueous  solutions,  free  from  impurities,  whereby 
its  usefulness  may  not  be  impaired,  and  from  which 
the  auric  chloride  formed  can  be  precipitated. 

Plattner  in  1856  proposed  what  is  now  known  as 
the  Plattner  system  of  chlorination,  which  he  accom- 
plished successfully  by  the  following  steps: 

1.  He  subjected  the  ores  to  a  roasting  process  for 
the  purpose  of  oxidation,  and  driving  out  of  the  ore 
those  substances  which  would  be  acted  upon  by  free 
chlorine. 

2.  He  leached  the  roasted  ores  with  water,   the 
ores  having  been  previously  saturated  with  chlorine 
gas. 


LIXiVIATlON  BY  CHLORINE  SOLUTIONS.         7 

3.  He  precipitated  after  filtration  the  auric  chloride 
thus  formed  in  the  second  step  by  means  ^of  ferrous 
sulphate. 

The  foundation  for  the  chlorination  process  of 
to-day  was  thus  built,  and  whatever  improvements 
have  been  made  are  along  the  lines  adopted  by 
Plattner. 

It  is  often  necessary,  in  treating  sulphides  contain- 
ing iron  and  other  base  metals,  to  chloridize  and 
roast  as  well.  This  is  especially  necessary  with  such 
ores  as  contain  much  sulphur,  arsenic,  antimony,  or 
other  such  volatile  compounds  which  should  be  dis- 
placed, and  which  simple  oxidizing  roasting  will  not 
accomplish. 

For  chloridizing  roasting,  common  salt  (NaCl)  is 
mixed  with  the  ore  (as  explained  under  the  heading 
Roasting,  p.  20),  converting  those  substances  not 
volatilized  into  soluble  and  insoluble  chlorides. 

Chlorine  has  a  remarkable  tendency  to  remove 
oxygen  from  the  oxides  of  metals,  and  so  long  as 
volatile  substances  are  combined  with  a  metal  during 
roasting  very  little  chlorine  escapes;  but  after  these 
are  displaced  by  heat  and  chlorine  the  chlorine  itself 
escapes — and  this  is  peculiar,  since  chlorine  is  very 
volatile. 

Due  observations  made  relative  to  the  amount  of 
volatile  matter  in  an  ore,  and  the  affinity  between 


8  THE    CHLORINATION  PROCESS. 

chlorine  and  that  substance,  allows  us  to  advance  one 
step  nearer  the  recovery  of  gold  by  chloridizing 
roasting  than  by  oxidizing  roasting. 

In  the  present  state  of  the  art  we  cannot  treat  ores 
by  wet  processes  as  they  come  directly  from  the 
earth;  or  could  ores  be  subjected  to  treatment  with- 
out roasting  for  chlorinating,  they  would  be  more 
readily  treated  by  some  cheaper  process;  conse- 
quently preliminary  treatment  is  a  necessity.  Again, 
the  chief  object  of  chlorination  is  to  recover  gold 
from  refractory  ores  where  cheaper  processes  will  not. 
We  therefore  find  chlorination  practised  upon  sul- 
phides, tellurides,  arsenides,  and  other  similar  ores 
•whose  combination  of  substances  can  be  broken  by 
oxygen  or  chlorine  with  the  assistance  of  heat.  As 
these  minerals  form  but  a  small  proportion  of  the  ore, 
they  are  concentrated  to  reduce  the  bulk  before 
treatment,  and  thus  save  waste  in  time,  fuel,  chemi- 
cals, and  capital.  Concentration  may  follow  amalga- 
mation unless  the  ore  be  too  refractory,  but  in  either 
case  crushing  of  some  description  is  preliminary  to 
concentration  and  subsequent  chlorination.  The 
foregoing  brief  description  of  the  object  sought  will 
;  allow  the  reader  to  follow  intelligently  the  process, 
which  should  proceed  as  follows: 

1.  Preparation  of  the  ore. 

2.  Roasting. 


LIXIVIAT20N  BY   CHLORINE  SOLUTIONS.          9 

3.  Roasting  appliances. 

4.  The  process. 

5.  Filtering. 

6.  Precipitating. 

7.  Refining. 

8.  The  cost  of  treatment. 

9.  The  plant  required. 

Under  the  above  headings  may  be  introduced 
details  not  of  minor  importance,  for  the  attention  to 
details  has  perfected  the  process  to  its  present  condi- 
tion of  usefulness. 

Precious-metal  mining  has  reached  the  stage  where 
capital  invested  judiciously  will  as  surely  bring 
returns  as  any  manufacturing  business  where  equal 
care  is  exercised.  The  difficulties  formerly  encoun- 
tered are  to  a  very  great  extent  overcome,  and 
chlorination  is  but  one  of  the  improvements  in  that 
direction.  We  do  not  mean  to  convey  the  idea  that 
all  difficulties  are  circumvented,  nor  do  we  mean  to 
convey  the  idea  that  lixiviation  is  able  to  do  more 
than  assist  in  recovery  of  gold,  and  we  particularly 
advise  parties  intending  to  enter  into  gold-mining  to 
beware  of  low-grade  milling  propositions  which  carry 
less  than  $10  per  ton  of  gold  where  chlorination  is  to 
be  practised. 

While  low-grade  bodies  of  ore  are  more  uniform 
and  continuous  than  "  bonanzas,"  there  is  a  limit  to 


10  THE   CflLORINATION  PROCESS. 

their"  low-gradeness, "  especially  if  refractory.  That 
one  low-grade  mining  company  with  a  large  mill  is 
able  to  treat  its  ore  at  a  good  profit  is  no  criterion 
that  another  large  mill  in  a  different  locality  can 
treat  the  ore  in  that  locality  at  all.  We  advise  in 
all  mining  enterprises  the  consultation  of  trained  min- 
ing engineers,  practically,  scientifically,  and  techni- 
cally educated. 


CHAPTER    II. 
PREPARATION  OF   THE  ORE. 

WERE  the  whole  mass  of  ore  to  be  treated  by 
chlorination  as  it  came  from  the  mine,  it  would 
be  economy  to  crush  the  ore  dry  preliminary  to 
roasting.  The  treatment  of  such  large  masses  of 
ore  would  require  an  immense  outlay  of  capital  for 
chlorinating  purposes,  together  with  increased  size 
of  the  plant  in  general.  For  instance,  if  but  one 
tenth  of  the  ore  carried  mineral,  the  mechanical 
arrangement  would  require  to  be  ten  times  as  large 
as  where  the  mineral  had  been  concentrated;  larger 
supplies  of  chemicals  and  fuel  would  also  be  needed. 
There  are  rare  instances  where  clean  silicious  ores 
could  be  treated  in  this  manner,  but  where  they 
occur  once,  the  probabilities  are  they  do  not  occur 
again  in  ten  thousand  instances. 

The  ore,  that  is,  the  vein  rock  containing  mineral, 
is  mined,  cobbed,  assorted,  loaded  into  suitable  con- 
veyances and  transported  to  the  mill,  as  the  first  step 

in    preparation.     In    some    instances   the    vein    rock 

ii 


12  THE    CHLORIttATION  PROCESS. 

will  not  show  mineral,  but  it  is  there;  and  in  such 
instances  the  rock  must  be  treated  with  the  mineral 
streak,  provided  it  carries  gold.  Vein  rock  between 
the  hanging  and  foot  walls  very  often  is  totally  bar- 
ren, and  again  will  carry  more  precious  metals  than 
the  mineral  streak  proper.  To  avoid  wear  and  tear 
upon  the  machinery  in  the  first  instance,  and  to  avoid 
loss  in  the  second,  judicious  assays  should  be  made 
and  recorded  of  all  vein  matter  broken  in  the  mine. 

When  the  ore  reaches  the  mill,  it  is  unloaded  over 
a  chute  which  has  an  inclination  of  40°  and  upwards. 
This  chute  has  for  its  floor  in  some  part  of  its  length 
a  number  of  iron  bars,  separated  by  spaces,  to  allow 
the  finer  ore  as  it  comes  from  the  mine  to  pass 
through  to  the  rolls,  while  the  coarser  passes  over 
the  bars  into  the  rock-crushers,  and  from  the  latter 
to  the  rolls.  These  screen-bars  are  termed  "  griz- 
zlies," and  assist  the  crushers  very  much  by  their 
separating  material  already  fine  enough  for  the  rolls, 
which  would  otherwise  interfere  with  the  amount  of 
ore  crushed  in  the  rock-breakers. 

The  most  suitable  rock-crushers  are  of  the  Blake 
and  Gates  type. 

The  former  has  been  longer  in  use,  so  that  by 
some  it  is  considered  to  be  the  only  crusher  of 
moment.  The  largest  size  of  the  Blake  we  believe 
is  No.  20,  with  an  opening  to  receive  rocks  13  X  30 


PREPARATION  OF   THE   ORE.  13 

inches  and  under,  which  it  can  crush  to  sizes  of  i|  to 
if  inches  in  diameter.  The  capacity  given  is  10  tons 
per  hour,  which  it  only  reaches  under  most  favorable 
circumstances.  A  fair  average  of  its  capacity  would 
probably  be  8  tons  per  hour. 

The  Gates  crusher  with  the  same  horse-power  can 
crush  double  this  amount  in  the  same  time.  Against 
the  Gates  crusher  is  its  weight,  and  while  it  crushes 
more  it  wears  more. 

For  fine  crushing,  two  small  crushers  are  better 
than  one  large  crusher,  as  setting  the  jaws  to  crush 
fine  hinders  the  amount  of  product  which  can  pass 
through.  However,  in  small  machines  the  opening  or 
mouth  for  the  reception  of  the  rock  is  smaller,  and 
would  require  finer  ore,  possibly  sledging,  in  order  to 
feed ;  consequently  a  large  crusher,  assisted  by  two 
smaller  ones  to  receive  its  product,  would  do  the  most 
economical  work. 

Mr.  Blake  puts  the  limit  of  economy  in  the  use  of 
multiple-jaw  crushers  at  No.  10  screen  (100  holes  to 
the  square  inch).  If  rolls  are  to  be  used,  and  then 
stamps,  there  is  not  so  much  saving  in  crushing  fine 
by  rock-breakers  as  would  appear,  as  all  ore  must  be 
thoroughly  dried;  and  if  lO-mesh  screened  ore  were 
admitted  to  the  stamp  battery  the  coarse  sand  would 
simply  pack,  and  the  agitation  necessary  to  keep  the 
ore  stirred  up  in  the  mortar  would  be  lacking.  For 


14  THE    CHLORINATION  PROCESS. 

the  above  reasons  multiple-jaw  crushers  for  fine 
crushing  can  be  dispensed  with  in  most  cases  where 
rolls  and  stamps  are  used,  and  the  jaws  of  the 
crusher  set  to  size  of  ore  which  will  work  well  in  the 
rolls.  It  is  advisable  to  use  crushers  whenever  possi- 
ble to  reduce  the  ore  for  the  rolls,  say  to  ij  inches 
diameter,  and  have  the  rolls  crush  this  to,  say,  J  inch 
diameter  before  admitting  to  the  stamps.  In  this 
way  the  product  of  the  stamps  may  be  increased. 

The  rolls  mentioned  are  cylinders  with  steel  tires 
turning  towards  each  other,  the  ore  passing  between 
them.  They  are  set  close,  with  heavy  springs  or 
swinging  pillow-blocks  so  arranged  as  to  give  if 
necessary  to  allow  large  pieces  of  rock  or  a  coupling- 
pin  or  piece  of  drill  steel  from  the  mines  which  has 
found  its  way  into  the  ore  to  pass  through  without 
breaking  the  rolls.  They  are  also  in  some  cases 
fitted  with  magnets  to  attract  any  iron  or  steel  which 
would  dent  the  faces  of  the  rolls. 

It  was  formerly  customary  to  gear  these  rolls,  but 
that  added  to  the  wear  and  tear;  they  are  now 
almost  universally  run  by  pulleys  and  belts.  If  the 
ore  which  passes  the  rolls  is  not  sufficiently  crushed, 
it  is  screened  and  returned  to  the  rolls  for  recrushing 
with  an  additional  supply  from  the  crushers. 

It  is  at  times  feasible  to  crush  fine  enough  with 
crushers  and  rolls  to  chlorinate  without  stamp-milling. 


PREPARATION  OF   THE    ORE.  1 5 

For  this  purpose,  however,  we  must  have  an  easily 
pulverized  rock,  and  return  all  the  product  which 
does  not  pass  the  desired  screen.  When  the  rock  is 
hard  this  method  is  destructive  to  screens,  wearing 
them  out  quickly  and  requiring  that  the  ore  be  free 
from  moisture  to  pass  the  screen.  For  fine  crushing 
the  most  satisfactory  arrangement  is  the  stamp-mill, 
especially  if  amalgamation  is  to  be  practised  before 
chlorination. 

The  action  of  these  crushing-machines  is  mashing, 
and  they  do  not  round  the  particles  of  ore  as  do 
certain  classes  of  pulverizers  which  pulverize  by  abra- 
sion. The  latter  class  are  not  suitable  for  gold  or 
silver  milling,  as  they  do  not  crack  the  grains  of  ore 
in  a  proper  manner;  however,  when  roasting  is  to 
precede  the  process  this  matter  is  not  of  such  moment 
as  for  amalgamation  or  cyaniding.  The  Huntington 
and  Chilian  mills  are  also  excellent  crushers  in  place 
of  stamps. 

If  free  silver  be  present  in  the  ores,  chloridizing 
roasting  followed  by  amalgamation  may  be  practised, 
or  the  ore  treated  first  to  recover  the  gold  by  chlori- 
nation and  afterwards  by  hyposulphite  of  soda  to 
recover  the  silver,  or  the  reverse ;  but  there  must  be 
in  each  case  silver  values  greater  than  gold.  Roasted 
gold  ore  does  not  amalgamate  readily,  while  silver 
gives  good  results,  Another  consideration  is  that 


1 6  THE    CHLORINATION  PROCESS. 

there  will  be  a  loss  of  gold  if  that  be  not  treated  first, 
as  it  is  generally  customary  to  add  chemicals  to 
brighten  the  mercury,  which  becomes  sickened  where 
roasted  ore  is  amalgamated. 

We  can  therefore  consider  all  silver  lost  where 
chlorination  is  practised  or  a  good  portion  of  the  gold 
lost  where  silver  is  recovered  first. 

The  ends  in  view  will  determine  the  crusher  to  be 
used.  Coarse  crushing  may  be  possible,  and  while 
objectionable  for  good  roasts  and  clean  concentrates, 
has  advantages  over  fine  crushing,  which  is  always 
attended  with  considerable  fine  ore  and  slimes  very 
inconvenient  in  filtering  or  drainage  to  wet  processes. 
With  chlorination,  however,  this  drawback  is  some- 
what removed  by  concentration  of  the  ore,  and  the 
subsequent  roasting  which  follows,  changes  the  shape 
of  the  mineral  particles  from  compact  to  porous, 
thus  facilitating  drainage  and  leaching. 

The  crushing  process  is  followed  by  concentration; 
that  is,  a  separating  of  the  mineral  from  its  gangue  or 
vein  rock  not  containing  mineral.  These  concen- 
trates are  usually  sulphurets  of  some  description, 
which  make  the  ore  refractory,  and  thus  necessitate 
chlorination ;  in  fact,  the  usefulness  of  the  process 
hinges  upon  this  class  of  ores. 

To  concentrate  the  tailings  from  the  crushers  they 
are  conducted  to  jigs,  where  they  are  washed  free 


PREPARATION   OF   THE   ORE.  1 7 

from  slimes.  The  sands  and  lighter  particles  are 
carried  from  the  jigs  to  vanners,  buddies,  bumping- 
tables,  or  other  similar  arrangements.  These  ma- 
chines, by  the  aid  of  water,  separate  the  lighter  par- 
ticles of  sand  from  the  heavier  particles  of  mineral: 
the  former  are  washed  away,  while  the  latter  are 
collected.  It  may  not  be  necessary  to  jig  in  all 
instances,  but  it  is  better  to  do  so  where  large  quan- 
tities of  sand  might  otherwise  go  to  the  tables,  and 
there  interfere  with  the  work.  When  fine  crushing 
is  practised  from  40  to  60  mesh  screen  (i.e.,  1600 
or  3600  holes  to  the  square  inch  respectively),  from 
30  to  60  per  cent  of  the  product  is  slimes,  and  unless 
these  are  separated  from  the  sands  and  concentrated 
separately,  they  will  be  lost.  To  accomplish  this  the 
ore  is  separated  into  fine  and  coarse,  each  carrying 
values.  This  product  is  now  carried  to  jigs,  which, 
having  more  even  sizes  to  deal  with,  are  able  to  treat 
with  a  greater  degree  of  certainty.  The  surplus 
products  of  the  jigs  then  go  to  the  tables — the  coarse 
to  one,  the  fine  to  another.  This  order  may  be 
somewhat  varied,  but  it  is  absolutely  necessary  for 
chlorination  that  the  concentrates  should  be  clean,  to 
give  the  best  extraction  results  and  the  least  loss  of 
values  in  concentration.  Slimes  will  adhere  to  the 
sands  unless  sufficient  water  is  used  to  separate  them ; 
and  as  they  usually  are  rich  in  precious  metal,  if  the 


1 8  THE    CHLORINATION  PROCESS. 

appliances  mentioned  do  not  answer,  they  should  be 
run  into  settling-tanks. 

The  cost  of  the  preparation  of  ore  is  against  the 
use  of  chlorination  at  times,  but  the  main  choice  of 
a  process  is,  other  matters  being  equal,  the  values 
saved. 

If  we  take,  for  example,  an  ore  carrying  2  oz. 
gold  ($40),  with  1 6  oz.  of  silver  ($9.60),  we  may  save 
95  per  cent  of  the  gold  and  lose  all  the  silver:  in 
money  this  loss  is  $12.08.  If  by  cyaniding  we 
recover  90  per  cent  of  both  values,  the  loss  amounts 
to  $4.96;  our  choice  would  therefore  naturally  be  the 
use  of  the  cyanide  process.  If  now  we  take  a  $40 
gold  ore  with  no  silver  value,  the  loss  by  chlorination 
with  95  per  cent  recovery  would  be  $2  against  $4  by 
the  cyanide  recovery  of  90  per  cent.  The  choice 
then  narrows  down  to  the  question;  can  chlorination 
be  practised  as  cheaply  as  cyaniding  upon  a  given 
ore  ?  No  direct  answer  can  be  given,  since  chlorina- 
tion has  more  preparation  of  ore  to  contend  with, 
and  if  in  our  example  the  margin  of  $2  is  consumed 
by  the  extra  manipulation  of  the  ore,  cyaniding  would 
be  the  best.  (See  Cost  of  Treatment.)  There  is  a 
limit  where  the  difference  in  favor  of  cyaniding  upon 
strictly  gold  ore  occurs,  which  is  that  point  where  the 
extra  expenses  of  chlorination  are  counterbalanced 
by  increased  extraction.  For  instance,  in  an  $80  gold 


PREPARATION  OF   THE   ORE.  IQ 

ore  with  95  per  cent  extraction  there  is  a  difference 
of  $4  in  favor  of  chlorination,  which  is  amply  suffi- 
cient to  favor  its  use  in  preference  to  cyanide  extrac- 
tion in  most  instances. 


CHAPTER    III. 
ROASTING  THE  ORE. 

HAVING  obtained  the  mineral  in  the  ore  as  con- 
centrates, by  the  preparation  noted  in  the  preceding 
chapter,  our  attention  must  be  directed  to  freeing 
that  mineral  from  the  base  metal  compounds  which  it 
contains.  To  accomplish  this  we  have  recourse  to 
oxidization,  which  in  our  case  is  the  application  of 
heat  with  oxygen  present  in  sufficient  quantities  to 
unite  with  the  base  metals,  forming  oxides  of  metals 
which  volatilize. 

Where  air  only  is  requisite  for  the  purpose,  the 
roasting  is  termed  "  oxidizing  roasting,"  but  where 
other  substances  must  be  employed  to  assist  decom- 
position, such  as  common  salt,  the  operation  is  termed 
"  chloridizing  roasting." 

The  chlorine  from  the  salt  (NaCl)  seems  to  answer 
a  twofold  purpose,  since  it  assists  not  only  oxidation, 
but  forms  combinations  with  metals  which  volatilize 
or  else  become  insoluble.*  To  roast  thoroughly,  the 

*Seep.  7. 

20 


ROASTING    THE   ORE.  21 

ore  should  not  be  subjected  to  a  high  heat  at  the 
commencement  of  the  operation:  in  fact  at  no  time 
should  it  fuse  or  melt,  but  the  heat  may  be  increased 
towards  the  close  of  the  roast. 

If  we  allow  fusion  in  roasting  we  cannot  attain  our 
object,  for  it  is  next  to  impossible  to  separate  some 
substances,  such  as  iron  and  sulphur,  which  have 
become  united  by  heat,  and  no  substances  which 
have  fused  are  as  susceptible  to  oxidization  as  before, 
while  its  action  or  destruction  of  the  leaching  com- 
pounds are  nearly  as  great  as  when  they  were  in  the 
raw  state. 

Certain  substances  admit  of  oxidization  and  will 
volatilize  to  a  certain  point,  after  which  it  becomes 
almost  impossible  to  oxidize  them  more;  at  this  stage 
chlorine  assists  on  account  of  its  great  affinity  for 
metals,  and  still  further  reduces  the  amount  of  the 
remaining  impurities. 

Yet  with  the  assistance  of  chlorine  it  is  not  possi- 
ble to  free  some  metals  from  impurities  such  as  iron 
sulphides.  The  greater  quantity  of  sulphur  is  readily 
volatilized,  but  the  last  2  per  cent  of  sulphur  from 
an  ore  containing  above  10  per  cent  sulphur  will 
require  as  much  labor,  care,  and  expense,  if  not 
more,  than  the  first  8  per  cent. 

When  sulphur  has  been  removed  to  the  lowest 
limit,  which  is  about  one  quarter  of  one  per  cent,  the 


22  THE    CHLORINA  TION  PROCESS. 

ore  is  said  to  be  "  roasted  dead,"  since  it  is  in  such  a 
condition  that  we  cannot  further  reduce  it  without 
great  expense,  and  is  practically  in  such  a  form  its 
injury  to  the  leaching  process  can  be  tolerated. 

The  affinity  of  sulphur  for  other  substances  than 
oxygen  modifies  the  process  of  roasting  somewhat,  as 
is  seen  where  chlorine  in  the  presence  of  hot  silver  is 
reduced  to  silver  chloride,  or  where,  when  arsenic  is 
to  be  evaporated,  carbon  is  added  to  the  mixture, 
producing  a  combination  more  easily  evaporated  than 
the  oxide. 

Chloridizing  roasting  deals  chiefly  with  sulphur 
compounds,  with  the  object  in  view  of  freeing  the  ore 
from  that  compound ;  no  doubt  there  will  be  met  in 
practice  other  difficult  substances  to  be  eliminated. 
It  must  be  borne  in  mind  that  roasting  is  not  melting, 
and  that  the  latter  must  be  avoided  if  it  be  the  object 
to  accomplish  a  good  roast,  and  high  extraction  by 
chlorination.  The  following  substances  are  of  fre- 
quent occurrence,  consequently  their  action  in  roast- 
ing is  worth  consideration. 

Iron  cannot  by  any  means  be  freed  entirely  from 
sulphur.  Roasting  will  reduce  it  to  about  8  per 
cent,  chloridizing  roasting  to  about  0.25  per  cent,  at 
which  point  elimination  ceases. 

Sulphide  of  zinc  (zinc  blende)  is  slow  to  oxidize, 
and  is  not  purified  of  all  its  sulphur.  It  further  vola- 


ROASTING    THE  ORE.  1$ 

tilizes  freely,  and  carries  off  the  precious  metals  asso- 
ciated with  it  when  exposed  to  moderate  heat. 

Copper  sulphides  can  be  readily  freed  from  their 
sulphur  by  slow  roasting  at  a  moderate  heat,  with  or 
without  salt. 

Tellurides  act  in  a  manner  similar  to  zinc. 

The  sulphide  of  bismuth,  being  easily  fused,  is 
difficult  to  oxidize,  since  it  melts  at  a  low  tempera- 
ture. 

Galena  is  nearly  as  difficult  to  oxidize  as  bismuth 
on  account  of  its  fusing  at  low  temperature. 

Mercury  and  silver  are  readily  liberated  from  ore 
by  heat.  The  former,  however,  volatilizes  at  a  low 
heat,  and  requires  great  care  when  roasting  to  avoid 
this. 

Mercury  used  to  collect  gold  and  silver  as  amalgam 
is  retorted  by  volatilization  and  the  fumes  condensed 
in  a  cooled  vessel  without  much  loss  of  quicksilver. 

Silver  is  readily  converted  into  silver  chloride  by 
roasting  with  salt.  This  salt  is  insoluble  in  water, 
but  dissolves  in  solutions  of  hyposulphite  of  soda. 
The  Russell  process  is  based  upon  this  fact. 

Sulphides  of  antimony  are  difficult  to  oxidize, 
because  extremely  fusible. 

The  sulphides  of  nickel  are  easily  oxidized,  forming 
pure  oxides.  The  same  applies  to  cobalt. 

Arsenic  sulphides  are   readily   oxidized,   both  the 


24  THE   CHLORINATION  PROCESS. 

arsenious  and  sulphurous  acids  forming  volatile  com- 
pounds. 

Phosphorus  cannot  be  freed  from  iron  by  roasting; 
neither  can  titanic  acid. 

Arsenic  is  also  difficult  to  remove  from  iron. 

Fine  ore  roasts  quicker  and  better  than  coarse;  but 
ore  not  sized,  that  is,  mixtures  of  coarse  and  fine  ore, 
offer  uneven  surfaces  to  the  action  of  heat  and  oxy- 
gen, one  getting  in  the  other's  way,  so  to  speak.  To 
assist  in  exposing  the  various  surfaces  of  the  ore  to 
the  reagents,  they  are  stirred  or  rabbled,  either  by 
hand  as  in  plain  reverberatory  furnaces,  or  by  fixed 
rabbles  as  in  moving  hearth-furnaces,  or  by  movable 
rabbles  as  in  the  Spence  furnace,  or  by  falling  from 
shelves  as  in  revolving  furnaces.  This  rabbling  also 
performs  another  office:  it  avoids  fusing,  sintering,  or 
melting  of  the  ore  during  roasting  by  its  agitation. 

The  roasting  ore,  after  moisture  has  been  driven 
entirely  from  it  by  heat,  becomes  like  so  much  quick- 
sand, flowing  rather  than  hanging  together  in  a  pasty 
mass. 

It  is  not  always  necessary  that  ore  should  be  ex- 
tremely fine:  in  some  instances  such  fine  ore  would 
be  a  hindrance  rather  than  a  help,  since  it  would 
cause  more  labor  in  preparation,  and  greater  care  in 
roasting  and  also  in  filtering.  The  principle  to  be 
governed  by,  is  to  crush  only  to  that  coarseness  which 


ROASTING    THE   ORE.  2$ 

will  roast  readily,  and  yield  the  greatest  per  cent  of 
gold  to  chlorine.  The  filtering  is  a  secondary  matter, 
for  when  once  the  auric  chloride  has  been  obtained 
in  solution  the  gold  may  be  recovered  from  it,  even 
if  time  is  long  drawn  out  in  the  process.  As  noted 
under  preparation  of  ore  (p.  16),  ores  undergo  a  com- 
plete change  in  physical  structure  when  roasted ;  they 
are  also  liable  to  be  converted  into  non-refractory 
ores;  at  any  rate,  they  are  not  refractory  to  chlorine 
solutions.  Mr.  Daggett  says:  "  The  leaching  of 
roasted  ores  is  quicker  done  for  any  process  than  the 
leaching  of  raw  ores,  unless  the  soluble  salts  formed 
in  roasting  have  received  a  prior  leaching,  when  it  is 
about  the  same  as  for  raw  ores." 

This  is  a  measure  true  in  chlorination,  where  some- 
times difficulty  in  filtering  occurs  when  fine  slimes 
prevent  the  passage  of  the  liquor.  In  other  leaching 
the  ore  packs  to  such  an  extent  that  it  is  next  to 
impossible  to  drain  at  all,  not  more  than  a  quarter  of 
an  inch  per  hour  being  recorded  with  raw  ore,  while 
with  ores  which  do  not  form  a  clayey  compact  mass 
impervious  to  water  filtration  is  comparatively  rapid. 

Where  the  base  metals  present  in  the  ore  are  in 
excess  of  the  volatile  substances,  it  may  be  policy  to 
hasten  oxidation  by  adding  small  quantities  of  iron 
pyrites,  and  allow  the  sulphur  driven  off  to  combine 


26  THE   CHLORINA  TlOfr  PROCESS. 

with  base  metals  before  adding  salt  for  chloridizing 
roast. 

Formerly  it  was  thought  necessary  to  have  a  cer- 
tain percentage  of  sulphur  in  the  charge  for  its  effect 
in  chloridizing.  This,  Mr.  Bruckner  thinks,  is  a  mis- 
take. In  his  judgment  the  iron  pyrites  is  added  for 
volatilizing  the  arsenic  and  antimony,  and  for  pre- 
venting the  formation  of  arseniates  and  antimoniates 
of  silver  and  gold  which  resist  chlorination ;  very  little 
sulphur,  or,  in  its  absence,  quartz,  is  sufficient  to 
evolve,  when  in  contact  with  a  very  small  percentage 
of  salt,  the  chlorine  necessary  for  chloridizing  all  the 
silver  contained  in  the  charge,  after  the  arsenic  and 
antimony  have  been  driven  off.  Thus  an  addition 
of  from  I  to  10  per  cent  of  iron  pyrites  is  in  most 
cases  required  for  the  success  of  the  roasting  process. 
Ores  which  are  apt  to  cake  at  a  low  temperature  are 
mixed  with  10  to  20  per  cent  of  rich  tailings.  Ores 
containing  lime  and  alumina  have  to  be  mixed  with 
silicious  ores  so  that  silica  shall  be  in  excess.  (Dr. 
R.  W.  Raymond,  A.I.M.E.,  1885.) 

Ores  containing  considerable  sulphur  in  them  must 
have  care  to  keep  down  the  heat  which  the  burning 
sulphur  creates,  and  salt  should  not  be  added  at  this 
stage,  since  the  heat  of  the  burning  sulphur  may  be 
sufficient  to  volatilize  the  chlorine  and  cause  a  loss  of 
gold  as  well.  When  salt  is  added  at  the  commence- 


ROASTING    THE  ORE.  2? 

ment  of  a  roast  in  the  reverberatory  furnace  a  loss  of 
gold  occurs,  but  not  to  so  great  an  extent,  and  some- 
times not  at  all  if  the  salt  is  added  on  the  hot  hearth 
or  end  of  the  roast  with  not  more  than  two  per  cent 
of  sulphur  present. 

At  the  end  of  the  roast  soluble  chlorides  are  formed 
as  readily  as  when  mixed  with  the  ore  at  the  com- 
mencement. 

Prof.  Christy  of  the  University  of  California  ascer- 
tained that  sulphides  roasted  without  salt  lost  little 
of  their  precious  metals,  but  that  with  3  per  cent  salt 
added  the  loss  in  one  instance  was  30  per  cent  of  the 
gold  and  50  per  cent  of  the  silver.  This  very  great 
loss  he  attributes  to  high  temperature  and  the  tellu- 
ride  in  the  ore. 

Plattner  from  his  experiments  states  "  that  loss  of 
silver  increases  with  the  temperature  in  roasting, 
with  the  looseness  or  porosity  of  the  ore,  and  the 
freedom  with  which  silver  combines  with  other  sub- 
stances.'* "  The  loss  also  increases  with  time  of 
roasting."  He  used  artificial  mixtures  for  his  roast- 
ing tests  to  ascertain  the  volatility  of  gold,  such  as 
arsenides  and  sulphides.  "  His  conclusions  were,  that 
a  loss  of  gold  can  take  place  only  in  oxidizing  roast- 
ing/ when  the  operation  is  carried  on  so  rapidly  that 
fine  particles  are  carried  off  mechanically." 

Prof.  Christy  and  others  coincide  with  him  in  hk 


28  THE   CHLORINATION  PROCESS. 

deductions  with  regard  to  oxidizing  roasting,  but  add 
that  "  he  does  not  seem  to  have  been  aware  of  the 
volatility  of  gold  in  chloridizing  roasting."  Kustel  is 
said  to  have  recorded  a  loss  of  20  per  cent  of  gold  in 
oxidizing  roasting  of  certain  tellurides  of  gold  and 
silver,  attributing  the  loss  to  the  volatilization  of  tel- 
lurium; and  says:  "If  salt  is  present  during  roasting, 
the  chloride  of  tellurium  volatilizes,  and  it  is  possible 
that  this  volatilization  causes  gold  to  volatilize  as 
well/'  Mr.  C.  H.  Aaron,  Prof.  Christy  says,  was  the 
first  to  publish  anything  definite  on  the  losses  of  gold 
in  chloridizing  roasting.  He  pushed  the  roasting  pur- 
posely with  and  without  salt;  the  salted  ore  upon 
assay  was  found  to  contain  but  one  half  as  much  gold 
as  the  unsalted. 

Mr.  C.  A.  Stetefeldt  gives  an  account  of  his  inves- 
tigations with  chloridizing  roasting,  and  found  the 
losses  to  be  from  42.8  to  93  per  cent  of  the  total 
gold.  He  also  states  "  that  volatilization  of  gold  no 
doubt  takes  place  with  copper  chlorides,"  but  adds, 
as  Mr.  Butters'  experiments  show,  "  that  copper 
chlorides  are  not  essential  to  produce  this  loss." 
"Mr.  C.  Butters  found  that  with  gold  ore  free  from 
copper  no  loss  occurred  in  oxidizing  roasting,  but 
when  chloridizing  roasting  was  undertaken  in  a  muffle 
with  5  per  cent  salt,  there  was  a  loss  of  from  68  to  85 
per  cent  of  the  gold."  '  He  found  also  that  an  in- 


ROASTING    THE   ORE.  2$ 

creased  quantity  of  salt  up  to  10  per  cent  caused  no 
increased  loss  of  gold." 

Prof.  Christy  has  shown  by  a  large  number  of  ex- 
periments, the  results  of  which  are  published  in  his 
admirable  paper  contributed  to  the  American  Insti- 
tute of  Mining  Engineers,*  that  muffle  tests  with  salt 
gave  a  higher  percentage  of  gold  loss  when  the  salt 
was  added  at  the  end  of  the  roast  rather  than  at  the 
commencement.  This  being  the  reverse  of  practical 
experience  in  continuous  roasting,  he  accounts  for  it 
as  follows:  "  Where  batch  roasts  are  made  and  the 
whole  lot  of  ore  is  kept  at  the  same  temperature 
throughout  (as  in  a  muffle),  when  the  gold  chloride 
has  once  formed  and  left  the  batch  of  ore,  that  is  the 
last  of  it.  At  the  end  of  a  roast  more  gold  being 
exposed,  the  addition  of  salt  produces  a  greater  loss 
of  gold." 

"In  practice  with  continuous  roasting,  if  the  salt  is 
mixed  throughout  the  ore  from  the  start,  a  continu- 
ous volatilization  goes  on  and  likewise  loss,  where,  on 
the  other  hand,  if  salt  be  added  at  the  finish,  or  hot 
end  of  the  furnace,  the  long  surface  of  unsalted  cooler 
ore  condenses  the  chloride  of  gold."  "  Part  of  this 
cooler  ore  is  giving  off  sulphurous  acid  gas,  and  this 
with  steam  from  the  burning  fuel  offers  excellent 

*Vol.  xvn.  p.  43. 


3O  THE    CHLORINATION  PROCESS. 

means  for  the  reduction  of  chloride  of  gold  in  the 
furnace;  but  the  most  efficient  means  is  in  the  pyrites 
themselves." 

The  factors  entering  into  "  salt  roasting  "  are  salt, 
temperature,  and  time. 

The  amount  of  salt  necessary  for  a  chloridizing 
roast  will  depend  upon  the  amount  of  oxidation  re- 
quired, and  this  must  be  governed  by  the  ore.  The 
less  salt  used  the  better,  but  enough  must  be  used  to 
drive  off  the  sulphur  and  form  silver  or  base  metal 
chlorides ;  the  leaching  which  follows  will  then  give  a 
high  percentage  of  recovery. 

Aaron  says  that  salt  should  be  below  4  per  cent. 
Butters,  Stedefelt,  and  Prof.  Christy,  and  the  deduc- 
tions of  others  from  practical  work,  bear  out  Mr. 
Aaron's  statement. 

Four  per  cent  of  salt  is  equivalent  to  80  Ibs.  to  the 
ton  of  ore.  That  amount  of  salt  is  probably  more  than 
required  for  driving  off  impurities,  and  what  cannot 
unite  with  impurities  present  will  unite  with  gold  to 
form  gold  chloride;  but  if  more  than  sufficient  be 
present,  chlorine,  being  readily  volatile,  will  escape 
with  loss  of  precious  metal.  A  few  tests  with  the  ore 
will  determine  the  percentage  of  salt  most  advisable 
for  roasting  with  an  ore. 

Temperature  has  been  mentioned  previously  as  of 
importance  in  chloridizing  roasting;  we  therefore  re- 


ROASTING    THE   ORE.  31 

fer  to  it  again.  It  is  necessary  in  continuous  roasting 
that  at  first  slow  heat  and  low  temperature  should 
take  place;  as  the  ore  moves  forward  towards  the 
fire  the  heat  increases  from  the  burning  sulphurous 
acid  gas;  when  nearer  to  the  fire,  the  sulphur  fumes 
having  ceased,  the  heat  is  increased.  Had  the  tem- 
perature been  as  great  at  the  commencement  as  at 
the  finish,  the  ore  would  have  fused  and  matted;  but 
the  volatile  and  combustible  gases  having  been  driven 
off  at  low  red  heat,  there  is  not  much  danger  of 
fusion  on  the  hot  hearth  at  a  cherry-red  heat.  The 
loss  of  gold  from  volatilization  with  salt  commences 
at  100°  C.  and  increases  until  it  reaches  its  maximum 
at  250°  C. 

Below  red  heat  the  loss  diminishes,  but  increases 
to  a  maximum  above  melting  heat. 

Prof.  Christy  gives  the  following  losses  of  gold  in 
a  stream  of  chlorine  gas  with  different  temperatures: 

At  incipient  redness  the  standard  loss  was  0.05  per 
cent  in  half  an  hour. 

At  a  low  red  heat  it  is  o.  10  per  cent. 

At  a  cherry-red  heat  it  is  0.25  to  0.35  per  cent. 

At  incipient  yellow  it  is  0.40  per  cent. 

At  melting  heat  it  is  0.50  +  per  cent. 

These  results  show  the  proper  regulation  of  tem- 
perature to  be  important  in  chloridizing  roasting;  also, 
that  the  proper  place  for  salt  is  at  the  end  or  hot 


32  THE   CHLORINATION  PROCESS. 

hearth  of  the  roast;  furthermore,  that  at  that  end  an 
over  supply  of  salt  is  not  beneficial,  since  the  chloride 
of  sodium  with  other  impurities  might  produce  fusion 
by  making  a  flux. 

It  has  been  ascertained  that  loss  of  gold  may  occur 
from  volatilization  when  a  high  heat  is  maintained  for 
a  long  time  if  salt  be  mixed  with  the  ore.  From 
the  foregoing  it  may  be  surmised  that  increased  tem- 
perature does  not  assist  roasting  with  salt,  and  conse- 
quently time  cannot  be  gained  except  at  the  expense 
of  gold,  and  afterwards  leaching. 

Time,  again,  as  a  factor  in  chloridizing  roasting  de- 
pends upon  the  amount  of  sulphur  or  other  volatile 
impurities.  While  roasting,  as  stated,  can  be  forced 
up  to  a  certain  limit  by  mechanical  contrivances 
and  automatic  furnaces,  "dead  roasting  "  cannot  mate- 
rially be  hastened. 

The  amount  of  impurities  may  be  so  small  and  so 
readily  reduced,  that  mere  contact  with  heat  will 
allow  reduction,  but  generally  time  and  patience 
are  required. 

In  one  instance  we  have  ore  oxidized  from  30  to 
7  per  cent  of  sulphur  by  falling  slowly  through  the 
furnace;  in  another  instance  we  find  it  requires  32 
hours  to  "  roast  dead  "  from  a  30$  sulphur  ore,  and 
at  times  it  may  require  even  more  than  this. 

In  chlorination  so  much  depends  upon  the  roasting 


ROASTING    THE   ORE.  33 

that  no  specified  time  can  be  stated  that  will  be 
applicable  to  every  ore.  By  hastening  the  operation 
we  run  chances  of  ruining  the  roast  by  melting;  and 
also  loss  of  the  metal  we  desire  to  save,  when  tel- 
lurides,  arsenides,  and  zinc  sulphides  are  volatilized. 
The  Austin  process  of  pyritic  smelting,  which  is  prac- 
tised under  certain  circumstances  to  utilize  the  heat 
of  the  burning  sulphur  and  melt  the  metal  to  a  matte, 
fully  illustrates  the  heat  which  may  be  evolved  from 
pyritic  ores,  and  which  in  our  case  must  be  avoided. 
The  whole  process  of  chlorination  depends  upon  the 
care  taken  to  obtain  a  uniform  and  complete  roast. 

The   deductions  we   may  make   as   far  as   regards 
pyritic  ore  roasting  are: 

1.  Sulphur  can  be  removed  in  part  by  heat,  more 
fully  by  a  plentiful  supply  of  air  with  heat,  and  prac- 
tically eliminated  by  salt  and  heat. 

2.  Sulphates  can  be  decomposed  by  air,  heat,  and 
chlorine,  combined  or  separate;   but  sulphides  must 
be   converted   into  sulphates  by  heat  with  air,   and 
finally  with  heat,  air,  and  chlorine  gas,  in  our  case. 

3.  Fusion  or  sintering  prevents  further  desulphuri- 
zation  or  formation  of  sulphates,  therefore  oxidation 
or  chloridizing  roasting. 

4.  Loss  of  gold   is  not  apt  to  occur  in  oxidizing 
roasting  where  care  is  used,  while  with  salt  added  to 
the  ore  at  the  commencement  instead  of  the  end  of 


34  THE   CHLORINATION  PROCESS. 

the  roast  it  is  considerable,  even  when  great  care  is 
taken. 

Prof.  Christy  found  by  experiment  that  the  loss  in 
the  latter  instance  was  308  times  greater  than  in  the 
former. 

5.  Salt  should  be  added  at  the  end  of  the  roast 
after  sulphur  has  ceased  to  be  evolved,  and  in  propor- 
tions which  have  by  experiment  proved  to  be  suffi- 
cient. 

Loss  of  weight  occurs  when  ore  is  roasted  by  the 
volatilization  of  volatile  compounds.  The  roasted 
ore  is  consequently  richer  in  precious  metal  per  ton 
than  raw  ore.  To  make  one  ton  of  roasted  ore 
from  33  J-  percent  sulphur  ore  will  require  1.3  tons 
of  raw  ore.  Salt  will  add  a  trifle  to  the  weight  of 
the  ore,  but  the  other  impurities  driven  off  will 
counterbalance  that. 

If  the  raw  ore  assays  $15  per  ton  in  gold  and  no 
loss  occurs,  the  roasted  ore  should  assay  one  third 
more,  or  $20  per  ton. 

The  theory  of  roasting  is  explained  as  follows: 

1.  The  conversion  of  sulphides  into  sulphates. 

2.  By   raising  the   temperature   the  sulphates  are 
decomposed  in  the  following  order  of  the  metal  com- 
pounds: 

Iron,  copper,  silver,  nickel,  zinc,  lead. 

Mr,  A.  Theis  gives  the  following  simple  plan  for 


ROASTING    THE   ORE.  35 

testing  the  degree  of  roast  in  the  furnace:  "  A  small 
portion  of  the  ore  from  the  furnace  is  boiled  in  water 
and  stirred  with  a  bright  iron  rod.  The  least  stain  of 
sulphur  on  the  rod  will  show  that  roasting  is  not 
complete." 

Second  Test. — Ferricyanide  of  potassium  will  indi- 
cate the  absence  of  any  ferrous  salt.  A  portion  of 
the  roasted  ore  is  boiled,  and  if  any  ferrous  salt  be 
present  in  the  ore  a  blue  coloration  will  be  given  to 
the  liquor  on  the  addition  of  ferricyanide  of  potas- 
sium. This  only  indicates  a  dead  roast  as  far  as  iron 
is  concerned. 


CHAPTER   IV. 
ROASTING-FURNACES. 

EFFICIENT  roasting-furnaces  must  allow  easy  con- 
trol of  the  heat,  abundant  access  of  air  to  the  hot 
ore,  and  rapid  removal  of  the  products  of  combustion. 
These  specifications  require  large,  flat  hearths,  that 
the  ore  may  be  spread  out  thin,  otherwise  small  ore 
charges  would  be  necessary ;  large  throats  leading  to 
stacks;  and  small,  shallow  fireplaces,  that  the  fire  may 
be  easily  regulated.  Should  the  draft  not  be  suffi- 
cient to  remove  the  products  of  combustion  quickly, 
artificial  draft  must  be  resorted  to,  or  the  stack  made 
higher  and  larger  in  the  flue.  This  forced  draft  will 
require  the  construction  of  dust-chambers,  and  if 
much  arsenic  or  other  volatile  substances  are  present 
they  will  be  necessary  under  all  circumstances,  as  the 
current  of  hot  air  will  carry  off  the  fumes  and  dust, 
and  with  them  fine  gold,  either  mechanically  or  chem- 
ically. 

Full  details  of  the  various  furnaces  invented  and 
discarded,  with  their  good  and  bad  points,  is  beyond 

the  province  of  this  work;  we  will  therefore  confine 

36 


ROA  S  TING-FURNA  CES.  37 

our  attention  to  those  which  are  in  common  use  and 
the  causes  which  have  led  up  to  their  adoption. 

Where  one  furnace  will  give  entire  satisfaction  in 
one  locality  it  will  not  in  another,  and  the  chloridiz- 
ing  roasting  for  the  chlorination  process  requires 
much  more  careful  work  than  the  same  roasting  for 
the  Russell  process,  or  for  the  amalgamation  of  silver 
ores  after  roasting.  Still,  at  times  it  is  within  the 
province  of  chlorination  to  use  the  mechanical  fur- 
naces to  great  advantage  where  low  percentages  of 
base  metals  and  sulphides  are  in  the  ore;  but 
where  they  are  above  8  per  cent,  "  dead  roasting  " 
is  a  difficult  matter,  and  we  have  no  authentic 
accounts  which  will  allow  us  to  assert  that  for  our 
purpose  mechanical  roasting  above  that  limit  can  be 
accomplished. 

There  are  two  classes  of  furnaces  to  choose  from — 
those  favored  by  mechanical  engineers,  and  those 
liked  by  metallurgists. 

At  this  date  inventors  realize  that  the  sale  of  a 
furnace  will  not  lead  to  success  in  their  line;  conse- 
quently they  wish  to  test  the  ore  to  be  roasted,  when, 
if  their  furnace  proves  efficient,  they  will  guarantee 
it  to  do  the  work  at  a  certain  cost  per  ton. 

Roasting  sulphides  commenced  with  pile-roasting 
in  the  open  air.  This  method  oxidized  a  portion 
of  the  pile,  smelted  another  portion,  and  answers 


3«  THE   CHLORINATION  PROCESS. 

to-day  where  matte  is  wanted,  but  is  not  advisable 
for  chlorination.  The  assortment,  the  grading,  the 
sledging,  handling,  and  carting,  and,  finally,  the  re- 
roasting  of  fully  one  third  the  pile,  made  inventors  in 
Wales  look  for  some  method  of  oxidizing  roasting 
which  would  do  better  and  cheaper  work  in  less  time. 
The  result  was  the  reverberatory  furnace,  which  has 
been  modified,  enlarged,  and  improved  in  shape  as  the 
objects  to  be  attained  became  better  known.  They 
were  first  constructed  as  in  our  illustration,  where  the 


Fig.1 


charge  was  roasted  and  withdrawn  into  a  cooling  pit ; 
from  this  construction  they  were  lengthened  to  one 
long  hearth;  and,  finally,  steps  were  added  to  assist 
in  working  the  charges  forward,  as  it  became  evident 
nothing  could  be  gained  by  increased  heat  in  the 
roast. 


ROA  S  TING-FURNA  CES.  39 

The  rabbling  or  stirring  next  attracted  attention. 
What  is  known  as  the  Spence  furnace  is  an  English 
invention  of  Peter  Spence.  These  have  rakes 
attached  to  a  rigid  frame,  which  are  operated  by 
automatically  reversing  the  engine.  The  adjustment 
is  such  that  ore  is  admitted  to  the  furnace  at  the 
same  rate  it  is  discharged,  and  falls  from  hearth  to 
hearth  as  desired.  The  clogging  and  wear  of  the 
rakes  was  an  objection,  but  this  is  said  to  be  over- 
come by  cooling  the  rake. 

The  American  modification  of  the  Spence  furnace 
is  known  as  the  O'Hara.  In  this  furnace  the  rakes 
are  moved  forward  by  an  endless  chain.  It  is  pro- 
vided with  two  hearths,  and  the  rakes  are  attached  to 
carriers  which  extend  through  slots  in  the  side  walls 
and  which  move  the  chain,  thus  being  away  from 
the  heat  and  sulphurous  acid  of  the  burning  and 
roasting  ore.  This  furnace  will  roast  40  tons  per  day 
down  to  7  per  cent  of  sulphur.  The  wear  and  tear 
upon  the  mechanical  parts  is  high  in  spite  of  the  pre- 
cautions taken  to  obviate  it. 

The  Pearce  Turret  furnace  has  hollow  arms  moved 
by  simple  machinery  situated  in  the  open  space 
within  the  annular  bed.  These  arms  are  cooled  by 
air,  which,  passing  through  them,  is  forced  against 
the  rakes  and  on  the  ore. 

The  Spence  furnace  built  with  steps  is  continuous 


4O  THE  CHLORINATION  PROCESS. 

in  its  action,  as  many  as  five  steps  and  hearths 
being  used. 

Mr.  Peters  gives  an  account  of  the  furnaces  which 
he  built  at  Butte,  Montana,  as  64  X  16  feet  outside, 
with  four  hearths,  each  14  X  15  feet  wide  in  the 
clear.  The  ore  is  charged  from  a  hopper  upon  the 
hearth  farthest  from  the  fire  in  36oo-lb.  charges, 
and  in  that  position  will  become  a  bright  red  in  two 
hours.  Each  furnace  calcines  1 1  tons  ore  in  24  hours, 
from  30  to  below  4  per  cent  sulphur,  and  consumes 
2  cords  pine  wood,  only  the  labor  of  two  men  being 
required.  A  similarly  constructed  furnace  is  used  at 
the  Treadwell  mill  in  Alaska,  where  dead  roasting  is 
required. 

The  Brown  Horseshoe  furnace  is  a  somewhat  later 
invention,  and  does  good  work  on  low-sulphur  ores. 

To  obtain  a  "  dead  roast  "  from  high-sulphur  ores 
we  are  obliged  to  leave  the  mechanical  furnaces,  and 
either  use  them  in  preliminary  roasting,  followed  by 
the  use  of  the  reverberatory  for  finishing  roast,  or  else 
discard  them  for  the  reverberatory  entirely. 

The  disadvantages  in  a  measure  overcome  by  the 
mechanical  rabbling  of  the  Spence  and  other  furnaces 
can  be  further  overcome  by  the  revolving  furnaces  of 
the  Bruckner  type,  or  the  revolving  pan-furnace  with 
fixed  rabbles,  but  in  either  case  the  dead  roasting 
must  be  done  in  the  reverberatory. 


ROA  S  TING-FURNA  CES.  4 l 

The  shaft  furnaces,  such  as  the  Stetefelt,  are  fairly 
good  for  chlorination  of  silver  ores,  but  will  not  answer 
for  the  gold  chlorination  process. 

Ores  carrying  a  high  percentage  of  arsenic,  zinc, 
tellurium,  bismuth,  antimony,  and  a  low  percentage 
of  sulphur  can  be  sufficiently  roasted  in  the  mechani- 
cal furnaces  at  a  low  cost,  but  the  advantages  of  these 
furnaces  for  a  high  percentage  of  sulphides  in  the  ore 
have  yet  to  be  demonstrated. 

The  old  reverberatory,  like  the  beehive  coke  oven, 
seems  to  hold  its  own  against  all  comers  as  far  as 
uniformity  and  product  is  concerned. 

The  reverberatory  furnace  is  so  well  known  among 
professional  men,  the  description  would  be  omitted 
were  it  not  the  object  of  this  book  to  reach  the 
student  and  non-professional  man  not  so  well  in- 
formed. 

In  our  illustration,  Fig.  I,  //"represents  the  hopper 
through  which  ore  is  charged  to  the  hearth  O.  On 
this  hearth  it  is  spread  out  in  a  thin  uniform  layer. 
The  hopper  is  provided  with  a  cover,  which  is  closed 
as  soon  as  the  oven  is  charged.  G  is  the  fire-box 
provided  with  grate-bars,  fire-doors,  and  ash-pit. 
The  fuel  usually  burned  is  wood,  as  it  is  free  from 
sulphur  and  ash,  which  might  in  a  measure  reach  the 
ore,  and,  furthermore,  a  wood  fire  is  more  readily  con- 
trolled. The  flame  from  the  burning  fuel  is  separated 


42  THE   CHLORINATION  PROCESS. 

from  the  ore  on  the  hearth  by  the  bridge  B.  This 
bridge  also  raises  the  flame  against  the  fire-brick 
arch,  which  reflects  it  down  upon  the  ore.  It  is  this 
reflected  or  turned  back  heat  which  gives  this  style  of 
furnace  its  name. 

As  the  flame  approaches  the  stack  it  becomes 
cooler,  and  for  that  reason  the  arch  pitches  towards 
the  hearth  gradually  until  it  meets  the  throat,  where 
after  passing  the  ore  it  goes  through  a  straight  flue 
and  up  the  chimney.  The  apertures  DD  are  working- 


Fig.  2 


doors  through  which  the  ore  is  spread  upon  the 
hearth  as  well  as  stirred  during  roasting.  E  is  the 
discharge-pit  for  the  calcined  ore. 

By  reference  to  the  figure  it  may  be  seen  that  con- 
siderable heat  might  be  saved  if  the  furnace  was 
longer,  or  longer  with  steps,  the  under  side  of  the 
throat  of  the  first  furnace  being  raised  to  answer  as  a 
fire-bridge  for  the  second  in  the  latter  instance. 

With  the  same  object  in  view  return-flues  are  built 


ROA  S  TING-FURNA  CES.  43 

underneath  the  hearth;  again,  one  furnace  is  built 
above  the  other  in  such  a  way  that  the  flame  passes 
from  the  first  furnace  up  and  back  over  the  hearth 
of  the  upper  one  and  then  up  the  stack. 

The  latter  style  of  furnace  is  termed  a  "  double- 
hearth,"  and  is  worked  as  follows:  The  top  is  used  for 
drying  the  ore,  after  which  it  is  dropped  to  the  first 
hearth,  upon  which  it  is  allowed  to  remain  twelve 
hours.  This  charge  is  then  dropped  to  the  lower 
hearth  and  a  new  charge  placed  in  the  upper.  After 
remaining  a  sufficient  time  on  the  lower  hearth  to 
allow  sulphur  fumes  to  cease,  salt  is  added  and  well 
stirred  into  the  ore.  The  time  required  for  this  roast- 
ing is  about  twenty-four  hours. 

The  furnace  has  the  advantage  of  utilizing  space,  of 
continuous  roasting,  of  creating  little  flue-dust,  and 
lessening  fuel  expenses.  The  size  of  such  a  furnace 
is  1 8  feet  long,  15  feet  high,  and  with  low  fire-brick 
domes  or  arches,  cast-iron  working-doors,  and  other 
appurtenances  will  cost  approximately  $1.50  per 
cubic  foot. 

Step-furnaces  are  built  as  in  our  illustration,  but 
one  furnace  is  raised  three  to  four  feet  above  the 
hearth  of  the  other.  The  ore  can  be  more  readily 
kept  separated  and  transferred  from  one  hearth  to 
the  next  in  this  furnace.  The  domes  can  be  brought 
lower  to  the  ore  as  well,  where  with  one  long  con- 


44  THE   CHLORINATION  PROCESS. 

tinued  reverberatory  this  cannot  so  readily  be  accom- 
plished. These  furnaces  are  about  60  feet  long  and 
15  feet  wide,  will  roast  3  tons  per  day,  and  have  a 
capacity  of  about  9  tons  of  raw  ore.  The  charges  are 
kept  separate  and  the  roasting  is  continuous.  One 
hearth  reverberatory  furnace  of  the  above  dimensions 
will  cost  $2  per  square  foot  of  ground-space;  step 
reverberatories  will  cost  $2.50  per  square  foot  of 
ground-space. 

The  working  of  such  furnaces  is  illustrated  as  fol- 
lows: 

Charged  at  8  A.M.  2500  Ibs.  raw  concentrates,  well 
dried  and  free  from  moisture. 

8.30  A.M.  Sulphur  commences  to  flame  on  the 
4th  hearth.  Fire  good. 

10  A.M.  Moved  charge  to  3d  hearth;  sulphur 
burning  briskly;  ore  hot  and  red.  Fire  almost 
without  fuel.  Charged  the  4th  hearth. 

2.30  P.M.  Moved  ore  to  2d  hearth,  large  quanti- 
ties of  sulphur  fumes  being  given  off.  Fire- 
place dark;  ore  red.  Moved  ore  from  the  4th 
to  the  3d  hearth;  sulphur  burning  freely. 
Charged  the  4th  hearth. 

3.30  P.M.  Combustion  of  sulphur  on  2d  hearth; 
decreasing  ore  swelling  slightly.  Fireplace 
dark.  Ore  on  3d  hearth  giving  off  large 


ROA  S  TING-FURNA  CES.  45 

quantities  of  sulphur;  ore  dark  red.  Ore  on 
the  4th  hearth  commencing  to  burn. 
4.30  P.M.  Combustion  of  sulphur  ceased  on  2d 
hearth;  added  3  per  cent  salt  and  mixed  well. 
Added  fuel  to  the  fire.  Ore  on  the  3d  hearth 
giving  off  large  quantities  of  sulphur  fumes; 
ore  on  4th  hearth  burning  briskly. 

5  P.M.     Charges    passed   from   2d   to    1st    hearth, 

3d  to  2d,  4th  to  3d,  and  new  charge  placed  on 
the  4th  hearth.  Fire  hot,  but  not  much 
chlorine  fumes  given  off. 

5.30  P.M.  Chlorine  fumes  ceased.  Fire  red-hot. 
Ore  swelling  slightly  on  2d  hearth;  sulphur 
burning  briskly  on  3d  hearth;  sulphur  just 
commencing  to  flame  on  the  4th  hearth. 

6  P.M.     Charge    drawn   from   the   1st  hearth  into 

the  pit.  Ore  moved  from  2d  to  1st  hearth, 
from  3d  to  2d,  from  4th  to  3d,  and  new  charge 
added  to  the  4th. 

In  this  continuous  manner  roasting  is  carried  on 
day  in  and  out. 

The  roasted  ore  is  not  immediately  removed  from 
the  pit;  it  is  left  there  some  time,  to  allow  chlorine 
fumes  to  pass  off. 

It  is  then  taken  to  the  cooling  floor  and  allowed  to 
become  cold,  when  it  is  dampened  and  elevated  to  the 
chlorinator  floor. 


46  THE   CHLORINATION  PROCESS. 

Wetting  down  roasted  ore  when  red-hot  is  likely 
to  cause  "  balling,"  in  which  condition  it  is  not  in 
proper  shape  for  chlorinating,  it  being  difficult  for 
chlorine  to  penetrate  the  balls  and  extract  the  gold. 

In  this  connection  we  may  also  note  that  raw  con- 
centrates should  be  kept  moistened  until  ready  for 
use,  when  they  are  dried  quickly;  otherwise  they  will 
be  likely  to  form  lumps,  which  are  difficult  to  roast 
thoroughly,  and  thus  again  a  loss  in  leaching  from 
imperfect  oxidation. 

The  revolving-hearth  furnace  is  an  iron  pan  which 
carries  the  ore  to  be  roasted.  It  is  geared  to  revolve 
horizontally,  and  has  fixed  rabbles  which  stir  up  the 
ore  as  it  comes  in  contact  with  them,  thus  exposing 
new  faces  to  the  heat.  These  furnaces  do  excellent 
work,  but,  as  before  noted,  are  not  able  to  do  "  dead 
roasting"  without  the  aid  of  a  small  reverberatory 
attached.  Prof.  Phillips*  gives  the  dimensions  of 
one  used  by  Mr.  Theis,  which  roasted  3  tons  of  con- 
centrates in  36  hours,  desulphurizing  pyrites  from  32 
to  0.25  per  cent. 

Diameter  of  the  hearth  or  pan 12  ft. 

Depth Sin. 

Height  of  dome  from  centre  of  pan   30  in. 

Furnace-wall  thickness 14  in. 

*  Vol.  xvii,  Trans.  A.  I.  M.  E. 


ROASTING-FURNA  CES.  47 

Fire-Lox 6  ft, 

Grate-surface,  2  X  3 6  ft. 

Fire-bridge  height 2  ft. 

Throat  length,  4  ft. ;  height  16  in.,  area  5  ft.  4  in. 
Working-doors  (one  each  side) 8  X  16  in. 

Reverberatory  attached : 

Length 14  ft. 

Width  inside 6  ft. 

Spring  of  arch 2  ft. 

Working-doors  (one  each  side) 8  X  16  in. 

Dust-chamber 4  x  3  X  20  ft. 

Spring  of  dust-chamber  arch 18  in. 

The  cost  given  for  roasting  by  this  furnace  one  ton 
of  concentrates: 

i  cord  wood,  at  $  1 .40 , $o.  70 

12  hours'  labor,  at  9  cts 1.08 

Motive  power , . .' , 0.25 

Cost  per  ton  of  concentrates $2.03 

As  it  requires  1.3  tons  of  such  concentrates  to 
make  one  ton  of  roasted  ore,  the  cost  of  roasting  one 
ton  of  ore  would  be  $2.64. 

Of  the  revolving-cylinder  furnaces  we  must  say  the 
same  as  for  the  other  mechanical  furnaces. 

The  Bruckner  furnace    as  originally  designed  was 


48 


THE    CHLORINATION  PROCESS. 


intermittent  in  discharge,  and  could  roast  but  one 
charge  at  a  time.  It  was  a  wrought-iron  cylinder, 
lined  with  4-in.  curved  fire-brick,  usually  1 8  to  22  feet 
long  and  6  to  8.5  feet  in  diameter. 

The  cylinder  had  two  orifices,  one  at  each  end ;  a 
manhole  for  the  receipt  of  the  ore,  and  its  discharge 
after  roasting.  The  motion  was  around  a  horizontal 

H 


IMPROVED 
BRUCKNER 

ROASTING  CYLINDER, 


axis,   but  as  improved  the  cylinder  turns  on  rollers 
geared  to  the  motive-power  shaft. 

The  two  contracted  orifices,  one  at  each  end,  are 
for  the  fire  and  escape  of  the  products  of  combustion. 
In  Fig.  3  an  improved  Bruckner  roasting-cylinder  is 
shown,  one  end  connected  with  a  movable  furnace, 
the  other  with  the  dust-chamber  D.  The  movable 


ROAS  7^ING-FURNA  CES.  49 

furnace  F  is  connected  with  the  cylinder  until  the 
charge  commences  to  blaze,  when  it  is  removed,  and 
only  reattached  to  complete  the  roast.  H  represents 
the  hopper  through  which  the  ore  is  charged. 

These  furnaces  are  better  adapted  to  the  roasting 
of  silver  ores,  in  which  case  they  will  roast  9  tons 
concentrates  inserted  in  one  charge  from  30  to  5  per 
cent  sulphur  in  24  hours. 

In  some  cases  the  cylinder  is  made  with  four  man- 
holes or  discharging-doors,  and  lined  with  fire-brick 
at  the  throats  and  red  brick  inside  the  cylinder. 

The  8.5  X  18.5  feet  cylinder  has  a  capacity  of  9 
tons,  and  weighs  45,000  Ibs. 

The  throat-linings  require  100  fire-brick. 

Fire-box,  500  fire-brick  and  4135  red  bricks. 

The  body  of  cylinder,  4300  bricks. 

If  movable  fire-box  is  used,  2800  more  fire-brick 
are  required. 

These  furnaces  when  used  for  chloridizing  roasting 
and  chlorination,  possess  the  advantage  of  allowing 
desulphurization  of  base  ores  before  salt  is  added, 
thus  obviating  one  difficulty  of  mixing  the  salt  with 
the  ore  at  the  commencement  of  the  roast.  The 
Bruckner  furnace  has  some  of  the  advantages  of  the 
reverberatory,  as  well  as  its  disadvantages,  increased. 
The  percentage  of  sulphur  may  be  decreased,  also 
the  heat  readily  controlled.  The  charge  may  be 


50  THE    CHLORINATION  PROCESS. 

retained  in  the  furnace  as  long  as  desired,  and  tested 
for  the  degree  of  roast  required. 

It  was  the  intention  of  Mr.  Bruckner  to  make  the 
furnace  automatic  and  continuous  by  placing  two 
furnaces  in  a  line,  but  stepped  one  below  the  other 
and  connected  by  a  feed-pipe.  The  fire  was  to  be 
made  at  the  lower  cylinder  and  circulate  through 
both,  thus  effecting  a  saving  in  fuel.  The  results  for 
comparison  between  Bruckner  and  reverberatory  fur- 
naces are  not  in  favor  of  the  Bruckner  for  dead 
roasting  for  a  high-sulphur  ore.  Where  low-sulphur 
ores  are  roasted,  the  advantage  must  be  with  the 
Bruckner. 

The  disadvantages  of  the  furnace  are: 

Care  must  be  taken  not  to  let  the  heat  get  "  too 
light  a  yellow,"  as  the  charge  will  ball. 

The  charge  has  to  be  heated  to  a  "  dark  yellow," 
and  become  sticky  to  avoid  flue-dust,  in  which  case 
it  balls  if  the  ore  be  fed  dry.  If  the  charge  is  wet 
the  ores  have  to  be  heated  to  a  dark  red,  which  is  the 
condition  for  flue-dust  to  form.  The  damper  must 
be  closed  and  a  hot  fire  kept  up  until  the  ore  is  a 
dark  yellow,  when  it  becomes  sticky  and  liable  to 
ball.  This  balling  is  a  great  drawback  to  chlorinating, 
as  the  ore  thus  roasted  must  be  recrushed  before 
being  placed  in  the  leaching-barrels;  but  in  such 
masses  it  cannot  be  properly  "  roasted  dead."  The 


ROA  S  TING-FURNA  CES.  5 1 

makers  say  it  is  possible  to  roast  ores  to  as  low  a  per- 
centage of  sulphur  in  the  Bruckner  furnace  as  in  a 
hand  reverberatory  furnace  if  the  operators  are  ex- 
perienced. 

For  desulphurizing  where  dead  roasting  is  not 
required  these  furnaces  are  well  liked,  one  firm  using 
168  of  them,  roasting  from  40  to  7  per  cent  sulphur 
with  167  Ibs.  of  coal  to  the  ton  of  ore  roasted.  There 
is  another  advantage  which  will  apply  to  almost  any 
furnace — a  further  decrease  in  the  cost  of  roasting  by 
the  use  of  fuel-gas.  The  simplicity  of  modern  fuel- 
gas  producers  allows  almost  any  fuel  to  be  converted 
into  gas  at  a  moderate  cost,  and  the  gas  fixed  or 
purified  to  avoid  any  deleterious  mixtures  with  the 
ore.  The  Mond  Gas  Producer  will  produce  60,000 
cubic  feet  of  gas  from  one  ton  of  inferior  coal,  coke, 
or  peat;  and  equally  good  results  may  be  obtained 
from  our  modern  producers,  such  as  Loomis  or  Tay- 
lor's, although  they  do  not  claim  as  much. 

The  Bruckner  furnace  has  been  improved  in  various 
details,  among  which  is  Clark's  oxidizing  and  desul- 
phurizing apparatus,  which  brings  the  air  in  contact 
with  the  ore  by  means  of  a  pipe  running  through  the 
furnace.  This  pipe  is  water-jacketed  to  keep  it  cool, 
while  air  is  forced  through  it  and  from  apertures  in  it 
to  the  cylinder.  This  principle  of  adding  more  air  is 
correct  in  every  detail,  as  naturally  what  air  is  admitted 


52  THE   CHLORINATION  PROCESS. 

to  the  throat  rises  to  the  top  of  the  cylinder  without 
coming  in  contact  with  the  ore,  and  so  out  the  stack. 
The  air  forced  in,  however,  through  the  pipe  would 
come  in  contact  with  the  ore  and  drive  the  sulphur 
fumes  from  the  burning  ore  to  the  top  of  the  cylinder 
and  allow  quicker  oxidation. 

These  furnaces  are  revolved  at  the  rate  of  100  feet 
per  minute,  and  should  not  be  above  that.  If  the 
charge  is  working  properly,  it  goes  half-way  up  the 
sides  of  the  furnace  and  slides  back;  if  above  this 
point  it  is  too  hot,  if  below  this  too  cold. 

The  cost  of  a  Bruckner  furnace  will  depend  upon 
freight  rates  from  some  manufacturing  point. 

A  20  X  8  furnace  will  weigh  45,000  Ibs.  It  is 
composed  of  iron  work  as  follows:  20  X  8  ft.  iron 
cylinder,  one  movable  fire-box,  one  60  ft.  X  30  in. 
iron  stack,  one  conveyor,  one  hopper  and  driving- 
gear,  driving-shaft,  countershaft,  pulleys,  pillow- 
blocks,  etc.,  complete. 

There  are  needed  3400  fire-brick,  10,000  red  brick, 
8  bbls.  fire-clay,  lime,  and  cement. 

The  labor  of  one  machinist,  two  bricklayers,  and 
three  helpers,  10  days. 

The  building  for  the  furnace  will  be  30  X  20  ft. 

The  cost  in  dollars  and  cents  will  approximately  be: 


ROA  S  TING-FURNA  CES.  5  3 

Cylinder  and  appurtenances $2,500.00 

Fire,  red  bricks  and  lime 335-OO 

Labor 210.00 

Building  30  X  20  at  $1.50  per  sq.  ft. 

of  ground  surface 900.00 


Total   $3,945.00 

To  which  must  be  added  the  freight. 

The  Howell  or  Oxland  furnace  is  a  modification  of 
the  Briickner.  The  Howell-Wh'te  differs  but  little 
from  the  Howell,  and  what  is  termed  the  improved 
White  is  a  trifle  different  yet.  They  are  all  rotating 
cylinders,  provided,  as  in  the  Howell-White,  with 
cast-iron  shelves,  which  raise  the  ore  and  let  it  drop 
back  as  it  rotates.  They  are  made  30  feet  long  and 
5  feet  in  diameter,  lined  entirely  throughout  with  4^- 
inch  curved  fire-brick.  The  Howell-White  has  but 
one  end  lined  with  these  bricks,  while  the  metal  on 
the  smaller  portion  is  exposed.  This  smaller  portion 
has  cast-iron  spirally  arranged  shelves,  which  showers 
the  ore  through  the  flames  as  it  turns. 

The  roasting  capacity  of  a  60  in.  X  27  ft.  furnace 
is  stated  to  be  from  30  to  45  tons  per  day.  The 
amount  of  brick  required  for  lining  furnaces  and  20 
feet  of  dust-chamber  is  28,000  red  brick  and  2700 
fire-brick;  fire-clay  is  also  required. 

The  Improved  White  roasting- furnace  differs  only 


54 


THE   CHLORINA  TION  PROCESS. 


in  being  a  long  cast-iron  revolving  cylinder,  lined 
throughout  with  fire-brick.  These  fire-brick  are 
curved  to  the  inner  circumference  of  the  cylinder,  and 
are  4^  inches  thick.  The  shelves  are  made  by  insert- 
ing key-brick  9  inches  thick  in  rows,  which  make  a 
projection  of  4^  inches.  These  shelves  run  nearly 
the  entire  length  of  the  cylinder.  The  greater  the 


Fig.  4. 

number  of  shelves  the  more  dust  is  made,  and  for  this 
reason  the  shelves  have  been  reduced  from  8  to  6. 

A  furnace  60  inches  in  diameter  and  30  feet  long 
will  require  28,000  red  brick,  9000  fire-brick,  and 
3  bbls.  of  fire-clay.  The  capacity  is  from  30  to  45 
tons  per  day.  The  weight  of  these  cylinders  is 
about  28,000  Ibs.,  including  bearing-wheels,  chairs, 
sole-plates,  gearing,  and  bolts,  exclusive  of  brick 
and  other  paraphernalia. 

These  cylinders  are  inclined  toward  the  fire  end, 
and  fed  at  the  upper  end  by  a  screw  feeder  situated 
as  shown  in  the  figure.  They  are  continuous  in 


ROA  S  T1NG-FURNA  CES.  5  5 

operation,  discharging  the  product  regularly  into  a 
pit  at  the  lower  end,  from  which  the  roasted  pulp  is 
withdrawn  as  required. 

Working  the  ore  towards  the  fire  is  the  true  theory 
of  perfect  roasting,  while  the  expenditure  of  power — 7 
to  12  H.P. — is  not  unreasonable  for  the  size  of  furnace. 

The  shelves,  as  mentioned,  create  considerable 
dust,  and  this  requires  mechanical  drafting  to  remove 
with  the  products  of  combustion. 

This  drafting  creates  another  difficulty:  dust  leaves 
the  cylinder,  as  does  fine  ore,  before  it  is  thoroughly 
chloridized.  We  have  noticed  also  that  if  salt  is 
used  the  chances  for  loss  of  gold  are  greatly  ad- 
vanced, when  the  salt  is  mixed  with  the  ore  at  the 
commencement  of  the  roast. 

To  roast  this  flue-dust  Mr.  Hoffman  added  a  fur- 
nace at  the  dust-chamber  end,  which  necessitated  two 
fires.  This  did  not  entirely  obviate  the  difficulty; 
and  in  some  instances  with  automatic  charging  one 
third  of  the  ore  went  into  the  dust-chamber  instead  of 
through  the  cylinder,  and  not  being  sufficiently  chlo- 
ridized had  to  be  reroasted  with  salt.  To  charge  out 
of  line  of  draft  one  or  two  patents  were  taken  out, 
and  in  another  instance  a  collar  was  inserted,  which 
formed  a  sort  of  chamber  which  possibly  stopped  the 
draft  and  some  of  the  dust. 

Mr.  Rumsey  claimed  for  this  latter  patent  that  the 


56  THE   CHLORINATION  PROCESS. 

chamber  formed  at  the  head  of  the  cylinder  held  the 
dust  until  roasted,  after  which  it  would  fall  to  the 
bottom  of  the  cylinder  and  move  with  the  heavier 
ore  down  the  cylinder  and  out  with  the  roasted  pulp. 

The  Hoffman  roasting-furnace  is  a  revolving  cyl- 
inder of  the  Bruckner  type,  with  a  fire-place  at  each 
end,  also  a  flue. 

The  flues  are  between  the  fire-places  and  cylinder, 
descending  to  dust-chambers  which  are  connected  with 
the  main  flue.  By  means  of  dampers  the  current  of 
air  can  be  reversed  to  go  in  either  direction,  and  thus 
expose  the  ore  charge  to  a  uniform  temperature. 

The  furnace  is  suitable  for  ores  which  require  a 
very  high  or  low  roasting  temperature. 

The  Brown  Horseshoe  Furnace  is  constructed  in 
the  shape  of  a  circle,  with  one  fifth  of  the  circle  left 
out.  The  cross-section  gives  an  arch  the  same  as  the 
common  reverberatory.  The  stirrers  are  mechanical, 
and  move  on  carriages  running  on  tracks  in  chambers 
on  each  side  of  the  furnace.  The  ore  is  charged 
and  discharged  automatically,  always  working  against 
the  hot  furnace.  The  fumes  may  affect  the  carriers 
and  moving  mechanism,  with  all  the  protection 
offered.  The  stirrers  make  the  round  of  the  furnace, 
and  then  stop  to  cool  off  in  the  one-fifth  open  space; 
they  work  automatically.  The  stirrers  do  not,  it  is 
claimed,  by  this  cooling  process  become  overheated. 


ROA  S  TING-FURNA  CES.  5  7 

The  points  of  superiority  are  claimed  for  this  fur- 
nace as  follows: 

1.  It  is  simple  in  construction,  requiring  no  more 
brick   or  iron  work  than   the  ordinary  reverberatory 
furnace  of  the  same  length  of  hearth. 

2.  It  is   50  per  cent  less  in  cost  than  any  other 
mechanically  stirred  furnace  of  the  same  capacity. 

3.  The  operating  mechanism  is  most  easy  to  man- 
age, and  least  liable  to  get  out  of  order. 

4.  The  carriages  moving  the   rakes,  standing  half 
the  time  in  the  open  air,  are  kept  thoroughly  cooled, 
and  are  at  all  times  perfectly  accessible. 

5.  Less  manual  labor  is  required,   one  man  on  a 
shift  taking  care  of  the  machinery  and  fires  for  a  fur- 
nace of  40  to  60  tons  daily  capacity. 

6.  The  hearth  being  on  one  plane,  no  dust  is  raised 
by  the  falling  ore,  and  no  loss  of  heat,  as  is  the  case 
with  furnaces  having  upper  and  lower  hearths. 

7.  The  feed  is  automatic,  introducing  any  required 
quantity  of  ore  with    the  passage  of  each  rake,  the 
amount    being    governed    by    a    counterpoised    lever 
which  weighs  each  charge. 

8.  The  machinery  is  perfectly  noiseless  in  opera- 
tion. 

9.  All  the  journals  that  are  exposed  in  any  manner 
to   the  heat  are   fitted  with   ball-   or   roller-bearings 
requiring  no  lubrication. 


$  THE   CHLOR1NATION  PROCESS. 

It  is  claimed  to  give  a  "  dead  roast  "  on  high- 
sulphur  ores.  The  author  has  had  no  opportunity  to 
thoroughly  inspect  the  working  of  this  furnace,  and 
no  data  but  the  maker's  to  go  by.  The  reports  re- 
ceived, however,  are  such  as  to  warrant  the  assertion 
that  this  furnace  is  a  great  improvement  in  mechani- 
cal roasting-furnaces,  and  worthy  further  investigation. 

Mr.  R.  P.  Rothwell  gives  an  account  of  one  of 
the  Howell-White  furnaces  used  by  him  at  Deloro, 
Canada,  in  roasting  ores  carrying  40  per  cent  arsenic. 
He  was  able  to  obtain  93  per  cent  of  the  gold  by  its 
use.  The  arsenic  fumes  being  very  dense,  flues  as 
well  as  dust-chambers  were  necessary.  The  forced 
draft  was  obtained  by  a  Guibal  fan. 

The  dryer  was  an  inclined  revolving  cylinder  48 
inches  diameter  at  the  mouth  and  36  inches  diameter 
at  the  throat,  with  a  conical  addition  at  the  throat 
making  its  total  length  22  feet.  Fire  passed  through 
this  cylinder,  from  which  the  ore  dropped  in  a  contin- 
uous manner  into  a  boot,  and  was  raised  by  elevator 
buckets  into  No.  I  roasting  cylinder. 

This  was  a  revolving  cylinder  30  X  5  feet,  was  sim- 
ilar in  lining  and  shelves  to  the  Howell  described 
above.  From  this  the  ore  ran  direct  into  a  second 
roasting  cylinder  20  X  4  feet,  lined  as  above,  and  in 
which  the  roasting  was  completed.  The  cost  of  roast- 
ing these  ores  is  given  as  60  cents  per  ton,  the  furnace 


ROA  S  TING-FURNA  CES.  5  9 

roasting  10  tons  in  24  hours.    This  ore  is  more  readily 
roasted  than  simple  sulphurets  would  be. 

The  obstacles  encountered  were  flue-dust,  loss  of 
fine  gold  carried  away  by  the  dust,  aided  by  the 
arsenic  fumes  and  volatilization. 


CHAPTER   V. 
THE    LEACHING   PROCESS. 

HAVING  prepared  the  ore  for  roasting,  and  by 
roasting  for  leaching,  we  come  to  that  stage  of  the 
process  where  the  outlay  from  the  foregoing  is  to  be 
recovered  with  interest.  In  the  first  chapter  we  dealt 
somewhat  at  length  upon  this  subject,  but  additional 
information  of  the  action  of  chlorine  and  its,  to  us, 
most  interesting  compound  of  gold  will  not  be  out  of 
place. 

Auric  chloride,  the  most  important  compound  of 
gold,  is  a  red  crystalline  mass  when  evaporated  to 
dryness,  soluble  in  water.  When  combined  with 
other  metal  chlorides,  it  forms  double  salts,  termed 
chloro-aurates,  the  general  formula  of  which  is  MCI, 
AuCl, ,  where  M  represents  an  atom  of  a  monad  metal. 
These  compounds  are  mostly  yellow  in  crystals,  but 
red  when  deprived  of  the  water  of  crystallization. 
It  seems  to  make  no  difference  to  chlorine  whether 
the  gold  is  cold  or  warm,  provided  no  other  impurities 

are  present  with  which  chlorine  unites. 

60 


THE  LEACHING  PROCESS.  6 1 

If  auric  chloride  (AuCl,)  be  heated  above  130°  C. 
subchloride  of  gold  is  formed;  and  by  warming  AuCl, 
or  gold  sponge,  in  AuCl3  a  double  chloride,  AuCl, 
AuCl3 ,  is  formed. 

Pratt  says  AuCl3  heated  in  chlorine  gas  forms  a 
higher  gold  chloride. 

Chlorine  gas  for  chlorination  is  usually  generated 
from  solutions  of  bleaching-powder  *  by  sulphuric 
acid.  We  may  consider  the  bleaching-powder  to 
have  the  formula  CaO  +  C14 ,  and  then  to  have  had  its 
Cl  displaced  by  the  air,  carbonic  acid,  or  oxygen,  and 
become  CaOCl3.  The  reaction  then  would  be: 

Au  +  CaOCl3  +  H2SO4  =  AuCl3  +  CaSO4  +  HaO. 

This  may  not  be  the  direct  action — that  point  seems 
to  be  in  doubt;  but  the  ultimate  reaction  is  as  given, 
a  gold  chloride  being  formed  when  nascent  chlorine  is 
liberated  by  sulphuric  acid  in  the  presence  of  gold. 

The  chlorination  process  is  based  upon  this  reaction. 

The  Plattner  process  of  chlorination  is  practised  as 
follows : 

The  ore  is  crushed,  concentrated,  roasted  with  salt, 

*  Bleaching-powder  is  composed  of  CaCla  -f  CaClaO3  ,  calcium 
hypochlorite. 

Roscoe  (vi.,  p.  176)  considers  hypochlorite  to  be  the  important 
factor.  The  result  for  our  purpose  is,  however,  ultimately  the 
same,  viz.,  the  production  of  AuCl3. 


62  THE   CHLORINATION  PROCESS. 

and  sifted  into  large  wooden  vats,  which  are  slightly 
raised  at  one  side  to  insure  drainage. 

There  is  a  filter-bed  (described  in  Chapter  VI)  in 
the  bottom  of  these  tanks,  through  which  the  liquor 
containing  the  auric  chloride  is  drained,  and  collected 
for  precipitation  (see  Chapter  VII,  p.  81). 

The  vats  are  of  a  size  to  suit  the  operator — say  9 
feet  in  diameter  and  3  feet  deep,  or  12  feet  in  diam- 
eter and  4  feet  deep.  The  depth,  however,  shouH 
not  exceed  4  feet,  unless  mechanical  contrivances  are 
employed  to  remove  the  tailings  from  the  vats  after 
leaching.  These  vats  should  be  lead-lined,  although 
this  is  not  absolutely  necessary,  but  will  prevent 
leakages  and  possibly  loss  of  gold  chloride  in  solu- 
tion. 

This  process  requires  about  four  days,  and  on  that 
account  four  leaching-vats,  to  allow  of  one  being 
cleaned  and  charged  daily.  The  charge  for  a  9  X  3 
ft.  tank  is  4  tons,  and  for  a  12  X  4  ft.  tank  7  tons,  of 
roasted  concentrates. 

In  order  to  sift  the  concentrates  into  the  tank  so  as 
to  lie  loosely  and  avoid  packing,  they  are  slightly 
dampened,  with  enough  water  to  form  a  ball  in  the 
hand  when  squeezed,  which  will  crumble,  however, 
when  the  hand  is  opened  and  pressure  removed.  This 
looseness  allows  the  chlorine  gas  to  permeate  the  ore, 
which  it  could  not  so  readily  do  were  it  packed  in  the 


THE  LEACHING   PROCESS.  63 

tank.  A  coarse  screen  over  the  tank  is  used  for  this 
sifting,  and  the  tanks  nearly  filled  with  ore,  but  space 
enough  left  to  assure  the  ore  being  covered  with 
water  after  gassing.  The  gas  is  generated  in  an 
apparatus  (usually  two  generators),  which  causes  it 
to  flow  into  the  bottom  of  the  tank  at  two  opposite 
points  and  work  upwards  through  the  ore.  This  gas- 
sing continues  until  ammonia  held  over  the  ore  gives 
off  the  dense  fumes  of  ammonium  chloride;  the  tanks 
are  then  covered  with  lids,  and  the  lids  luted  with 
clay  to  prevent  any  escape  of  gas.  This  operation 
requires  about  three  hours. 

The  tank  being  charged  with  chlorine  gas  is  left 
standing  with  the  gas  in  contact  with  ore  two  days. 
The  gas  is  not  forced  into  these  tanks  by  pressure, 
but  continues  to  generate  and  go  into  them  for  a  time 
after  the  covers  are  placed  on  them.  The  amount 
of  gas  required  is  fairly  well  known  for  one  class  of 
ores,  as  is  the  amount  of  chemicals,  so  that  the  gener- 
ators are  charged  accordingly  to  supply  that  quantity. 

On  the  third  day  the  ore  is  leached  by  filling  the 
tanks  with  water,  which  immediately  dissolves  the 
gold  chloride  and  holds  it  in  suspension. 

The  gold  chloride  is  now  fixed,  and  can  only  be  pre- 
cipitated by  some  metal  salt,  as  metallic  gold.  This 
solution  is  now  filtered  off  by  removing  the  plugs  or 
opening  lead  spigots  at  the  bottom  of  the  vat.  Wash- 


64  THE   CHLORINATION  PROCESS. 

water  is  added  until  upon  testing  no  chlorine  appears 
to  be  present,  when  we  may  consider  the  leaching  to 
have  been  completed.  Water  is  added  in  sufficient 
quantities  to  keep  the  tank  full  during  this  process. 
This  leaching  process  requires  from  four  to  five  hours. 

G.  W.  Small  *  gives  an  account  of  tank  chlorination 
at  the  Plymouth  Mining  Co.'s  chlorination-works  in 
California,  where  the  ore  was  passed  through  a  35- 
mesh  screen,  1205  holes  to  the  square  inch,  which 
allowed  quick  leaching  and  a  recovery  of  95  per  cent 
of  the  gold.  The  liquor  from  the  tank  is  run  or 
pumped  into  settling-tanks,  where  sulphuric  acid  is 
added,  to  hold  in  suspension  any  impurities  which  the 
excess  of  water  has  dissolved  from  the  ore,  and  also 
to  allow  mechanical  impurities  in  suspension  to  settle 
in  these  tanks  rather  than  in  the  precipitation-tanks. 
After  settling,  the  liquor  is  drawn  off  into  precipita- 
tion-tanks, where  the  gold  is  thrown  down  as  a 
brownish  precipitate  by  ferrous  sulphate,  which  proc- 
ess will  be  explained  in  Chapter  VII. 

If  lime  or  talc  is  present  in  the  concentrates,  the 
ore  in  roasting  may  have  its  lime  converted  into  sul- 
phide of  lime.  This  sulphide  must  be  entirely 
decomposed,  or  the  leaching  in  the  vat  or  barrel  will 
prove  unsatisfactory. 

*  Trans.  A.  I.  M.  E".,  Vol.  xv.  p.  307. 


THE   LEACHING   PROCESS.  65 

The  chlorine  will  be  converted  into  calcium  chloride, 
and,  furthermore,  hydrogen  sulphide  will  be  evolved, 
which  will  precipitate  the  gold  from  the  auric  chloride 
solution  as  fast  as  formed  until  the  excess  of  chlorine 
gas  ceases.  This  will  be  lost  with  the  tailings, 
unless  reroasted  and  retreated. 

'*  To  use  the  Plattner  process  on  lime  ores,  no 
calcium  sulphide  should  be  present  in  the  material 
when  it  enters  the  leaching- vat.'* 

Whenever  the  lime  is  in  the  shape  of  chloride  of 
calcium,  leaching  can  occur  without  loss;  but  no  roast- 
ing test  will  determine  the  presence  of  calcium  sul- 
phide. 

If  carbonates  be  present  in  the  ores,  the  hypo- 
chlorite  of  calcium  in  the  bleaching-powder  will  be 
decomposed,  and  consequently  the  hypochlorous  acid 
will  remain  inert. 

The  Mears  process  had  for  its  object  a  saving  in 
time  over  the  Plattner  process.  It  is  so  similar  in 
details  with  the  Theis  process  that  the  two  can  be 
described  together.  The  chlorine  gas  was  generated 
for  the  Mears  process  outside  the  chlorinating-barrel, 
while  in  the  Theis  process  it  is  generated  inside  the 
barrel.  The  latter  process  has  entirely  excluded  the 
former,  as  far  as  barrel  chlorination  is  concerned,  on 
account  of  its  simplicity  and  its  removal  of  the 
objectionable  features  of  the  former. 


66  THE   CHLORINATION  PROCESS. 


BARREL   CHLORINATION. 


The  chlorinator,  Fig.  5,  was  with  the  Hears  process 
a  cast  or  sheet  iron  cylinder  capable  of  withstanding  a 
pressure  of  60  Ibs.  per  square  inch.  This  cylinder  was 
lined  with  sheet  lead  weighing  10  Ibs.  to  the  square 


Fig.  5. 

foot.  It  was  fitted  with  cast-iron  cylinder  heads, 
which  were  securely  bolted.  It  revolved  upon  a 
hollow  trunnion,  through  which  the  gas  was  forced  into 
the  interior  of  the  cylinder  by  means  of  a  pressure- 
pump.  This  trunnion  was  arranged  to  admit  at  one 
end  an  iron  lead-lined  gas-pipe,  termed  a  "  goose- 
neck," one  end  being  in  the  trunnion,  the  other  out- 
side, upon  which  was  a  pressure-gauge  to  record  the 


THE  LEACHING   PROCESS.  67 

pressure  inside  the  cylinder.  The  barrel  was  charged 
with  roasted  ore  and  chemicals,  then  rotated  from  15 
to  20  times  per  minute. 

The  Newberry-Vautin  process  used  compressed  air 
as  well  as  the  pressure  generated  by  the  chlorine  gas. 
The  air  was  to  be  compressed  in  wooden  barrels  to  100 
Ibs.  per  square  inch,  and  that  with  the  gas  pressure 
was  to  permeate  the  rock.  It  had  to  permeate  the 
rock  or  burst  the  barrel — we  think  the  latter,  as  the 
process  has  come  down  to  generating  the  pressure  to 
20  Ibs.  in  the  barrel,  and  this  pressure  is  proved  to 
be  necessary  on  account  of  the  preliminary  treatment 
of  the  ore. 

Mr.  Theis  ascertained  that  pressure  was  merely  an 
accompaniment  to  barrel  chlorination,  and  might  be 
as  readily  obtained  inside  the  barrel  as  out,  the  main 
object  to  be  attained  being  the  liberation  of  chlorine 
in  the  presence  of  gold,  and  under  such  conditions  as 
would  mechanically  compel  the  gas  to  attack  the  gold 
in  the  least  possible  time. 

He  therefore  discarded  the  generator,  gas-storage 
tank,  and  pressure-pump,  and  employed  chloride  of 
lime  for  the  chlorine  gas  and  sulphuric  acid  to  lib- 
erate it,  charging  these  chemicals  in  suitable  propor- 
tions directly  into  the  cylinder. 

This  enabled  him  to  do  away  with  the  hollow 
trunnion,  and  substitute  solid  shaft  trunnions  securely 


68  THE   CHLORINATION  PROCESS. 

bolted  to  the  heads  and  provided  with  tight  and 
loose  pulleys.  The  cylinder  for  a  one-ton  chlorinat- 
ing charge  was  made  42  inches  diameter  and  5  feet 
long.  The  "  goose-neck,"  which  was  continually 
leaking,  was  next  discarded,  and  in  its  place  was  sub- 
stituted a  lead  valve,  by  which  the  gas  in  the 
cylinder  was  tested  to  better  advantage,  thus  obtain- 
ing more  uniform  extraction. 

The  generation  of  gas  in  the  cylinder  is  tested  from 
time  to  time.  The  pressure-gauge  might  show  con- 
siderable pressure  in  the  cylinder,  but  there  was  no 
means  by  which  it  could  be  determined  whether  that 
pressure  was  chlorine  gas  or  some  other  gas.  In  case 
it  was  the  latter,  and  the  gas  shut  off  from  the  gen- 
erator, the  extraction  was  poor;  but  by  means  of  the 
lead  cock  chlorine  is  readily  detected,  and  if  free 
chlorine  is  not  found  present  on  testing,  a  time  suit- 
able having  elapsed,  more  lime  and  acid  are  added. 
It  can  be  seen  that  an  appliance  which  allows  the 
gases  in  the  cylinder  to  be  tested,  rather  than  by  an 
uncertain  pressure-gauge,  is  of  immense  importance. 

The  illustration  given,  Fig.  5,  is  a  one-ton  chlori- 
nating barrel  of  the  Theis  pattern.  The  cylinder 
may  be  cast  or  wrought  iron  with  flanged  ends.  The 
heads  are  cast  iron,  with  solid  cast  trunnions  to  rotate 
in  suitable  boxes.  These  heads  are  bolted  to  the 
cylinder,  and  made  perfectly  air-tight. 


THE  LEACHING   PROCESS. 


The  cocks  for  testing  are  shown  on  each  side  of  the 
manhole  H  at  C,  C. 

The  illustration  gives  the  barrel  with  a  geared 
flange  at  one  end,  and  driven  by  a  small  wheel  con- 
necting with  the  pulley-wheel ;  these  barrels,  however, 
may  have  the  trunnion  extended,  and  upon  it  loose 
and  tight  pulley-wheels  for  motive-power  attach- 


Fig.  5a.  Chlorinating  Barrell. 


Fig.  5b, 


ments.  They  may  also  be  turned  directly  by  belts 
passing  around  the  barrels.  With  barrels  larger  than 
three  tons  capacity  it  is  advisable  to  build  them  of 
wrought  iron  or  steel  boiler-plate,  capable  of  standing 
150  Ibs.  pressure  to  the  square  inch.  They  should 
also  rotate  on  tires  bolted  to  them,  as  shown  in  the 
Bruckner  furnace  and  Figs.  5#  and  b,  these  tires  re- 
ceiving their  revolving  motion  from  the  rollers  upon 


?O  THE   CHLORINATION  PROCESS. 

which  they  rest.  The  rollers  are  keyed  to  shafts  and 
connected  by  gears  to  motive-power  shafts. 

A  five-ton  barrel,  60  inches  in  diameter  and  9  feet 
long,  could  receive  six  charges  in  24  hours,  and  is 
therefore  capable  of  chlorinating  30  tons  of  roasted 
ore  or  concentrates  per  day. 

This  size  requires  no  more  attention  than  a  one-ton 
chlorinator,  and  is  consequently  less  expensive  to 
work.  There  is  no  specified  limit  to  the  length  of 
these  chlorinators :  they  can  be  made  up  to  20  feet,  and 
chlorinate  20  tons  at  a  charge.  Their  cost  in  such 
instances  would  not  be  as  much  as  4  five-ton  chlori- 
nators, while  their  capacity  would  be  120  tons  per  day. 
With  such  large-sized  chlorinators,  from  five  tons  up- 
wards, their  ends  should  taper  as  do  barrels — first,  to 
allow  the  liquor  to  drain  from  the  centre  of  the  barrel 
or  larger  diameter,  and,  second,  to  allow  of  smaller 
heads  which  can  be  handled  more  readily,  and  which 
if  subjected  to  high  pressure  need  not  be  reinforced 
by  braces. 

Fig.  5#  gives  an  outline  of  such  a  barrel,  with  heads 
made  of  f-inch  boiler-plate.  The  bolts  which  fasten 
the  heads  are  to  be  countersunk  inside. 

The  shell  is  f-inch  boiler-plate,  10  feet  long,  60 
inches  inside  diameter  at  the  centre,  and  50  inches 
inside  diameter  at  the  ends.  The  object  of  bolting 
the  cylinder-heads  is  to  allow  of  quicker  repairs  and 


THE  LEACHING  PROCESS.  7 1 

examination  of  the  lead  lining,  also  to  change  the 
filter-bed  with  more  facility.  The  lead  lining  is 
bolted  to  the  shell  by  rows  of  flat-headed  bolts  or 
lead  rivets  spaced  about  18  or  20  inches  apart.  The 
sheet  lead  is  dressed  back  against  the  iron  shell  with 
lead  hammers  or  wooden  mallets.  There  are  two 
manholes  instead  of  one,  as  in  Fig.  5;  size,  10  X  12 
inches.  The  Colorado  Iron  Works,  the  E.  P.  Allis 
Co.,  and  Fraser  &  Chalmers  make  a  specialty  of 
chlorinating  plants;  full  particulars  for  these  barrels 
can  be  obtained  from  them,  together  with  their  cost. 
The  transverse  joints  should  be  single-riveted,  the 
longitudinal  double-riveted,  since  the  cylinder  may 
be  called  upon  to  withstand  60  to  80  Ibs.  pressure. 

The  speed  per  minute  for  such  barrels  should  not 
be  over  100  feet. 

The  method  of  charging  these  barrels  is  as  follows : 
The  barrel  is  first  partially  filled  with  water;  the 
proper  amount  of  chloride  of  lime  is  added;  on  top  of 
this  the  roasted  ore  is  placed,  and  finally  the  proper 
amount  of  sulphuric  acid.  The  manhole  covers  are 
then  put  on  and  screwed  down  tight,  after  which  the 
barrel  is  set  in  motion.  The  water  first  put  in  the 
barrel  is  for  the  purpose  of  making  an  easy-flowing 
pulp.  The  lime  is  added  next,  that  it  may  more 
intimately  mix  with  the  ore,  which  latter  will  work 
through  it.  The  sulphuric  acid  is  added  last,  so  that 


?2  THE    CHLORINATION  PROCESS. 

little  gas  may  be  generated  before  the  covers  are  on 
and  the  barrel  set  in  motion,  and  also  as  a  precaution 
of  safety.  About  one  fifth  more  acid  is  added  by 
weight  than  chloride  of  lime,  the  object  being  to 
convert  all  the  lime  possible  into  calcium  sulphate,  to 
remain  on  the  filter  as  well  as  to  set  free  all  the  chlo- 
rine gas  from  the  chloride  of  lime.  When  lime  passes 
the  filter-boxes  a  bulky  precipitate  is  produced  in  the 
settling-tanks,  or  if  it  passes  the  latter  the  precipita- 
tion-boxes; an  excess  of  acid  in  the  chlorinator  will 
avoid  this. 

Mr.  Theis  advocated  and  practised  the  division  of 
the  chemical  charge. 

He  first  charged  one  half  the  amount  of  the  lime 
and  acid  into  the  ore  and  water  mixture,  then  closed 
the  chlorinator  and  rotated  it  three  or  four  hours, 
after  which  the  remaining  half  of  the  chemicals  were 
added,  and  the  chlorinator  rotated  two  or  three  hours 
longer.  By  this  method  a  possible  saving  of  chemi- 
cals may  occur,  for  if  the  ore  were  not  properly 
roasted  there  would  be  free  chlorine  in  the  cylinder, 
but  if  properly  roasted  very  little  will  be  free.  The 
time  required  for  this  process  is  from  four  to  eight 
hours,  between  five  and  six  hours  being  the  average. 

The  presence  of  free  chlorine  is  detected  by  the 
means  of  the  lead  cock  mentioned,  and  the  cylinder 
rotated  at  least  one  hour  after  its  detection  to  insure 


THE  LEACHING   PROCESS.  73 

proper  absorption  of  the  gas  by  the  water  and  metal. 
The  ore  is  then  discharged  upon  a  shallow  filter-bed 
below  the  barrel,  or  the  cocks  opened,  when  filtering 
is  to  take  place  from  pressure  applied  inside  the 
barrel. 

Aqua  ammonia  may  be  used  as  a  test  for  free  chlo- 
rine, as  stated  in  the  Plattner  process. 

The  cause  for  the  presence  of  gas  being  in  the  gen- 
erator, other  than  chlorine,  or  the  cause  why  chlorine 
gas  is  not  properly  absorbed,  may  be  traced  almost 
entirely  to  improper  roasting.  If  copper  be  present 
more  bleaching-powder  and  acid  are  needed,  and 
again  the  recovery  may  be  so  low  that  the  whole 
charge  must  be  dried  and  reroasted.  Suppose,  for 
example,  we  have  roasted  ore  which  we  know  assays 
$80  per  ton,  and  from  which  we  generally  recover  95 
per  cent  or  $76,  but  from  improper  roasting  we  are 
only  able  to  extract  80  per  cent  or  $64.  It  will  pay 
to  reroast  this  charge  and  rechlorinate  if  we  can 
recover  95  per  cent,  or  $15.20  of  the  $16  present  in 
the  tailings;  but  the  cost  of  chlorination  was  but 
$3.50  in  the  first  place,  and  by  double  work  it  has 
mounted  to  $7. 

With  proper  care  in  roasting  the  percentage  of 
recovery  by  barrel  chlorination  will  be  between  90 
and  98  per  cent  as  the  extremes  and  94  as  the  aver- 
age extraction. 


74  THE   CHLORINATION  PROCESS. 

Mr.  Rothwell,  one  of  the  first  to  employ  the  Hears 
process,  states  that  with  a  good  roast  40  Ibs.  per 
square  inch  was  given  in  the  cylinder,  but  with  a  poor 
•roast  but  25  Ibs.  per  square  inch  was  recorded  on  the 
pressure-gauge  in  a  certain  time.  This  may  be  ex- 
plained in  the  first  instance  by  the  gold  forming 
chloride  and  leaving  an  excess  of  gas,  in  the  second 
by  other  chlorides  being  formed  besides  gold. 


CHAPTER   VI. 
FILTERING. 

THE  chloride  of  gold  having  formed  in  the  chlori- 
nator,  water  is  added  to  make  a  liquid  pulp.  The 
barrel  is  then  revolved  a  few  times  to  wash  out  the 
chloride  from  the  ore,  and  the  pulp  dumped  direct 
upon  the  filter-bed.  The  barrel  is  now  washed  by 
water  and  a  few  more  revolutions  given  it,  this  wash- 
water  in  turn  is  added  to  the  filter-bed,  the  barrel  is 
now  ready  for  another  charge.  This  practice  may 
vary  somewhat:  the  liquor  may  be  decanted  from 
the  barrel  direct  into  the  filtering-tanks,  wash-water 
added,  and  the  ore  and  water  run  onto  the  beds;  or 
the  filtering  may  take  place  from  the  barrel  itself, 
if  provided  with  an  asbestos  or  sand  filter,  in  which 
latter  instance  pressure  may  be  applied  to  hasten  the 
filtering. 

Sand-filters  are  constructed  as  follows :  For  a  one- 
ton  charge  they  are  lead-lined  boxes  6  feet  wide,  8 
feet  long,  and  18  inches  deep;  for  five-ton  barrels 
they  are  8  feet  wide,  15  feet  long,  and  3  feet  deep. 

75 


7  THE    CHLORINATION  PROCESS. 

The  bottom  is  covered  with  clay  or  other  substance 
impervious  to  water  and  not  acted  upon  by  action  of 
acid  and  chlorine,  such  as  perforated  glazed  tile. 
The  bottom  has  a  fall  of  about  one  inch  in  8  feet. 
Upon  the  floor  clean  gravel-stones  \  to  I  inch  in 
diameter  are  placed  in  a  layer  about  2  inches  thick. 
Another  layer  of  finer  gravel-stones  is  placed  over 
this  about  I  inch  thick,  and  upon  this  finer  gravel 
about  I  inch  thick,  and  lastly  fine  clean  quartz  sand, 
making  a  filter  about  5  or  6  inches  thick.  To  pre- 
vent the  filter  from  getting  uneven  surfaces,  strips  of 
board  ij  to  2  inches  wide  are  laid  about  10  inches 
apart  on  its  surface.  This  filter  is  then  flooded  from 
below  upwards  until  the  water  stands  over  the  sur- 
face. The  pulp  from  the  chlorinator  when  discharged 
cushions  on  the  water  and  slats,  preventing  the  sur- 
face from  becoming  uneven,  also  the  filter  from  pack- 
ing, and  allows  the  pulp  to  flow  evenly  over  the  face 
of  the  filter-bed.  The  corks  or  rather,  plugs  from  the 
lower  end  of  the  filter-bed  are  now  removed,  and  the 
liquor  allowed  to  flow  into  the  settling-tank.  The 
filtering  requires  three  or  four  hours,  and  is  followed 
by  a  wash-water  of  from  150  to  300  gallons  per  ton  of 
ore,  or  until  no  reaction  is  given,  when  the  filtered 
water  is  tested  with  ferrous  sulphate  (FeSO4). 

The  reaction,  if  trichloride  of  gold  is  present,  would 
present  a  reddish-brown  precipitate  of  very  fine 


FILTERING.  77 

brown    gold,    which    reaction    may    be   expressed   as 
follows  :* 


2AuCl3  +  6FeSOe  =  2Au  +  Fe3Cla  +  2(FeaO3, 

The  filtrate  should  be  quite  clear,  and  the  filtering 
accomplished  as  speedily  as  possible.  So  long  as  the 
solution  shows  the  presence  of  free  chlorine,  when 
the  last  wash-  water  is  leaving  the  filter,  the  ore  has 
been  thoroughly  leached.  f 

For  each  barrel  there  are  four  filter-boxes,  so  that 
at  least  one  will  be  ready  for  use.  When  these 
filter-boxes  are  deep  or  have  been  used  some  time  the 
rate  of  drainage  is  not  satisfactory.  Instead  of  drain- 
ing at  the  rate  of  three  or  four  inches  per  hour,  the 
drainage  is  not  more  than  one  inch  or  less  per  hour. 
Mr.  E.  G.  Spilsbury  advocated  decanting  the  first 
water  and  the  barrel  wash,  and  only  dumping  the 
last  wash  and  ore  on  the  filter.  Prof.  Phillips  does 
not  approve  of  this  method,  and  considers  time  lost 
rather  than  gained  by  so  doing. 

Mr.  R.   P.  Rothwell  was  one  of  the  first  to  wash 

*See  Chapter  VII. 

f  Chlorine  water  decomposes  iodine  compounds,  setting  free 
iodine  when  not  in  excess.  A  dilute  solution  of  metallic  iodide 
mixed  with  starch  paste  acquires  upon  the  addition  of  a  little 
chlorine  water  a  blue  tint  at  once,  which  becomes  colorless  again 
upon  the  addition  of  more  chlorine  water. 

The  presence  of  chlorine  is  also  indicated  if  a  white  precipitate 
is  formed  by  a  few  drops  of  silver  nitrate,  AgNO3, 


78  THE    CHLORINA  TION  PROCESS. 

the  ore  in  the  barrel  and  decant  the  liquor;  the  ore 
when  thoroughly  washed  in  the  barrel  was  removed 
to  the  dump  by  a  stream  of  water,  not  going  into  the 
filter.  This  system  required  much  time  for  washing. 

The  next  step  was  to  dump  into  a  filter-box  and 
wash  under  pressure;  but  this  was  objectionable  be- 
cause of  channels  being  formed  in  the  filter-bed, 
through  which  the  wash- water  passed  instead  of  uni- 
formly percolating  through  all  the  ore,  making  the 
washing  very  irregular. 

Mr.  J.  E.  Rothwell  turned  his  attention  to  wash- 
ing and  filtering  in  the  barrel  under  pressure.  For 
this  purpose  he  had  drainage-holes  made  in  the  barrel 
on  one  side  and  on  the  other  holes  for  connection 
with  pressure-pump.  At  first  he  employed  an  as- 
bestos-cloth filter,  which  he  found  difficulty  in  ob- 
taining suitable  for  the  purpose,  and  high-priced. 
The  fibre  of  the  cloth  was  destroyed  by  the  pressure, 
making  the  process  expensive;  otherwise  it  was  a 
success. 

To  overcome  these  objections,  he  devised  and  con- 
structed a  sand-filter  inside  the  barrel,  which  lasts 
about  one  month,  and  then  has  to  be  replaced,  when 
subjected  to  a  pressure  of  20  to  30  Ibs.  per  square 
inch.  He  gives  a  description  of  this  filter  in  vol. 
LX  of  the  Engineering  and  Mining  Journal,  p.  274. 
Mr.  Rothwell  advocates  this  filter  which  works 


FILTERING.  79 

satisfactorily.  It  is  one  of  his  inventions  and 
we  believe  it  is  considered  worthy  of  universal 
adoption,  although  the  mechanical  construction  seems 
to  be  faulty  on  first  appearance;  but  when  the 
barrel  has  been  in  motion  a  while  the  swash,  which 
must  be  considerable  at  first,  will  probably  be  over- 
come by  the  ore  remaining  against  the  inner  periphery 
of  the  cylinder.  The  .question  of  time  versus 
economy  in  filtering  is  one  of  considerable  moment, 
and  yet  to  be  fully  demonstrated:  with  slow  filtering, 
time  is  consumed,  but  less  cost  entailed;  with  quick 
filtering,  less  time  is  required  and  more  cost  added  to 
the  process.  The  sand-filters  require  replacement  as 
well  as  the  filters  placed,  in  the  barrel,  but  they  are 
more  readily  repaired  and  at  much  less  cost.  They 
do  not  require  power,  but  power  after  installation  of 
a  plant  does  not  add  much  for  filtering,  and  is  not  a 
separate  expense;  it  belongs  to  the  barrel. 

The  Plattner  process  requires  a  filter  in  the  bottom 
of  the  tank,  in  which  the  ore  is  leached.  Upon  the 
floor  of  the  tank,  which  is  slightly  inclined  for  drain- 
age, are  laid  f-inch  strips  of  wood  about  one  foot 
apart.  On  top  of  these  and  at  right  angles,  with  one- 
inch  spaces  between  them,  6-inch  boards  are  placed. 
Upon  this  false  bottom  loose  pieces  of  quartz  rock  or 
clean  gravel  are  placed.  On  these  finer  pieces  in 
layers  until  the  top  layer  is  clean  fine  sand.  Upon 


So  THE   CHLORINAT1ON  PROCESS. 

this  sand,  at  right  angles  to  the  false  bottom,  boards 
are  laid  quite  close  together,  as  from  these  boards  the 
ore  must  be  shovelled  from  the  tank  after  leaching 
and  drainage. 

The  same  difficulties  will  occur  with  the  use  of  this 
filter  as  in  the  previous  mentioned  sand-filters,  which 
can  only  be  remedied  by  shovelling  out  the  quartz 
and  sand  and  laying  a  new  filter-bed. 


CHAPTER   VII. 
PRECIPITATION. 

THE  liquor  from  the  filter-beds  is  conducted  or 
pumped  to  storage-tanks.  If  the  liquor  be  muddy 
after  passing  the  filter-beds,  or  if  it  contains  impuri- 
ties in  suspension,  it  is  treated  with  sulphuric  acid  to 
precipitate  them  in  these  settling-tanks.  The  time 
for  the  liquor  to  remain  in  these  tanks  will  depend 
upon  the  clearness  of  the  solution ;  not  less  than  two 
hours  and  sometimes  24  hours  may  be  required. 
These  tanks  are  of  sufficient  size  to  hold  the  liquor 
from  one  day's  run  of  the  filters.  The  accumulations 
of  liquor  in  these  stock  tanks  is  drawn  off  as  desired 
for  the  precipitation-tanks. 

These  latter  tanks  are  usually  not  over  4  feet  deep 
and  6  feet  wide  by  8  feet  long,  and  of  sufficient  num- 
ber to  allow  the  precipitate  to  settle  three  days. 
Each  tank  of  the  above  size  will  hold  liquor  from  3 
tons  of  ore,  and  consequently  a  6o-ton  daily  plant 
would  require  60  such  tanks  and  occupy  a  floor-space 

of  4320  feet,  allowing  one  foot  between  rows.     This 

Si 


82  THE    CHLORINATION  PROCESS. 

floor-space  can  be  economized  by  making  the  tanks 
longer  and  wider,  but  they  should  not  be  made 
deeper.  With  vats  12  X  16  feet,  or  6  X  32,  or  8  X 
24,  the  floor-space  required  will  be,  with  one  foot 
between  tanks,  3600  square  feet.  When  made  deeper 
than  4  feet  cleaning  up  becomes  unhandy. 

Precipitation-tanks  are  lead-lined  to  avoid  leakage, 
and  lead-lining,  not  being  very  stiff,  requires  skilled 
work  to  avoid  leakage  when  tanks  are  made  large. 
After  the  precipitation-tank  is  nearly  filled  with  liquor 
from  the  stock-tanks  it  is  treated  with  ferrous  sulphate, 
with  the  following  reaction : 

2AuCl3+  6FeS04  =  2  An  +  Fe2Cl6  +  2(Fe3O3.3SO8). 

The  precipitated  oxide  of  gold  is  a  dark  reddish- 
brown  powder,  brought  about  by  an  interchange  of 
metals  in  the  presence  of  oxygen.  The  ferrous  sul- 
phate (copperas)  should  be  made  fresh  daily  for  use, 
it  being  converted  by  the  oxygen  of  the  air  into 
various  basic  sulphates  if  allowed  to  remain  exposed 
any  great  length  of  time.  Its  precipitating  qualities 
will  be  weakened  where  Fe2(SO4)3,  or  ferric  sulphate, 
is  formed  in  particular. 

To  manufacture  ferrous  sulphate  (FeSO4),  scrap 
iron  is  thrown  into  a  wooden  tank,  and  sulphuric  acid 
added  until  hydrogen  is  evolved.  After  standing 
some  time  the  liquor  is  drawn  off  for  use,  and  more 


PRECIPJTA  TION.  83 

water,  iron,  and  sulphuric  acid  added  for  the  next 
day's  solution. 

It  requires  about  three  days  to  settle  the  gold  pre- 
cipitate, which  is  formed  in  a  very  finely  divided 
brown  powder.  The  fluid  in  which  the  gold  is  sus- 
pended has  a  blackish-blue  color  by  transmitted  light. 

To  ascertain  if  precipitation  be  complete,  the  solu- 
tion is  tested  every  24  hours. 

The  absence  of  chlorine  and  a  sweetish  odor  are  fair 
indications  of  a  complete  precipitation,  but  for  a 
certainty  a  small  quantity  of  the  liquor  is  stirred 
thoroughly  in  a  beaker  with  FeSO4.  When  the  pre- 
cipitate in  the  tanks  has  settled,  the  liquor  above  is 
either  drawn  or  siphoned  off,  a  fresh  solution  from 
the  stock-tanks  admitted,  and  again  the  copperas 
solution  added.  At  stated  intervals  the  settled  pre- 
cipitate is  taken  up  from  the  precipitation-boxes  and 
placed  in  smaller  lead-lined  boxes,  where  it  is  settled 
again,  and  what  little  liquor  remains  siphoned  off. 
The  filtrates  are  then  well  washed  with  boiling  water 
until  free  from  iron  salts,  after  which  they  are  col- 
lected on  filters,  dried,  and  melted  into  bullion.  The 
bullion  can  be  raised  to  990  -f-  degree  of  fineness 
where  care  is  used  in  skimming.  Borax  and  soda  are 
usually  used  as  a  flux  in  this  melting. 

A  more  recent  method  introduced  by  Mr.  Werner 
Langguth  presents  some  advantages  over  the  above 


84  THE   CHLORINATION  PROCESS. 

method,  which  we  believe  will  in  time  supersede  the 
ferrous  sulphate  method.  A  description  given  by  him 
of  the  process  as  carried  on  at  the  Golden  Reward 
chlorination-works  is  found  in  vol.  XXI,  p.  314,  of 
the  A.  I.  M.  E.  Transactions.  The  disadvantages  of 
this  process  are:  Impure  bullion,  and  extra  work  in 
refining  on  account  of  the  gold  precipitate  being  in 
the  form  of  gold  sulphide;  also  that  other  impurities 
are  thrown  down  by  the  hydrogen  sulphide  used  for 
precipitating  the  gold ;  and,  finally,  the  intricate  ap- 
paratus suggested,  as  shown  in  Fig.  6. 

The  advantages  of  this  process  are  economy  in 
space  for  precipitating-tanks;  quickness  in  pre- 
cipitating, and  recovery  from  the  auric  chloride  solu- 
tion. 

The  trichloride  of  gold  solution  is  pumped  into  a 
tank,  PPT,  which  is  raised  about  25  feet  above  the 
filter-pump,  FP,  for  head.  We  do  not  see  the  advan- 
tage of  this  arrangement,  especially  when  compressed 
air  is  used.  This  tank  has  a  capacity  of  7000  or 
more  gallons,  say  of  a  size  10  X  12  X  12  feet,  made  of 
strong  two-inch  pine  plank,  lined  with  light  sheet 
lead.  The  generators,  G,  are  two  in  number,  made 
of  boiler-plate,  and  capable  of  standing  a  pressure 
of  150  Ibs.  to  the  square  inch.  Their  size  is  about 
2\  feet  in  diameter  by  4  feet  high.  The  shape  is 
cylindrical,  with  cylinder-heads  securely  bolted  to  the 


PRECIPITA  TlOtf.  85 

shell  and  provided  with  manholes.  One  generator 
is  for  the  production  of  sulphurous  acid  (SO,),  and 
has  an  iron  pan  on  a  tripod  for  the  reception  of  sul- 
phur, which  is  burned  in  this  generator  to  produce  the 
gas.  It  is  not  lead-lined. 


Fig.  6. 

The  other  generator  is  for  the  production  of  hy- 
drogen sulphide  (H2S),  similar  in  size  and  construc- 
tion, but  lead-lined,  and  has  no  tripod  and  pan. 

Both  generators  are  connected  with  air-pressure,  to 
force  the  gases  into  the  precipitation-tanks,  and  also 
with  a  discharge-hole  at  the  bottom  for  cleaning  out 
the  refuse  after  each  run. 

The  pressure-tank  (see  Fig.  6,  PT)  is  constructed 


86  THE   CHLORINATION  PROCESS. 

of  boiler-iron  to  withstand  a  heavy  pressure.  It  is 
cylindrical  in  shape,  size  4  X  4i  ^eet,  inside  measure- 
ment, fitted  with  manhole  and  proper  threaded  holes 
for  receiving  the  pipes  from  the  air-compressor,  and 
also  for  pipes  conducting  the  precipitants  from  the 
clean-up  to  the  filter-press. 

The  liquor  containing  the  precipitate  is  run  into 
this  pressure-tank,  and  from  that  into  the  filter-press, 
at  stated  times.  The  liquor  freed  from  the  precipi- 
tate or  the  liquor  above  it  in  precipitation,  is  run 
directly  from  the  precipitation-tank  into  the  filter- 
press  as  soon  as  the  precipitate  has  settled  sufficiently. 
A  lead  pipe  leads  up  from  the  generators  to  this  tank 
over  its  side  and  down  to  within  four  inches  of  the 
bottom,  when  it  turns  at  right  angles  and  runs  across 
the  tank.  This  pipe  has  holes  on  the  under  side 
about  one-eighth  inch  in  diameter  and  six  inches  apart. 
When  air  carrying  the  gases  is  forced  through  it,  the 
gases  rise  up  through  the  liquor,  and  by  the  agitation 
which  it  causes  soon  brings  the  liquor  in  contact 
with  it. 

The  chloride  of  gold  liquor  having  been  forced 
into  the  precipitation-tank,  roll  sulphur  is  placed  in 
the  iron  pan  of  the  sulphurous  acid  generator  and 
ignited.  Compressed  air  for  combustion  and  forcing 
the  sulphurous  acid  gas  into  the  tank  is  then  ad- 
mitted to  the  generator  The  object  of  this  gas  is 


PRECIP1TA  TlOtf.  87 

not  to  precipitate  gold,  but  to  destroy  any  excess 
of  free  chlorine  which  may  be  in  the  liquor.  This 
generator  forms  sulphurous  acid  by  oxidation  of 
sulphur  as  follows : 

S  -f  2O  =  SO2. 

The  reduction  of  free  chlorine  then  takes  place 
rapidly,  as  expressed  by  the  formula 

S0a  +  2d  +  2H20  =  H2S04  +  2HC1. 

This  changes  the  color  of  the  liquor  from  that  of 
yellow  to  clear  blue  in  a  short  time,  producing  a 
white  fog  of  sulphurous  acid  and  chlorine  gas,  which 
is  led  up  C,  a  ventilating  chimney.  When  no  trace 
of  chlorine  is  detected  (see  tests  for  chlorine,  pp.  63, 
73)  the  gold  is  precipitated  by  sulphuretted  hydrogen 
(H2S)  formed  in  the  other  generator  by  the  action  of 
sulphuric  acid  on  iron  matte  (iron  sulphide).  Hydro- 
gen sulphide  gas  is  forced  up  through  the  solution  by 
compressed  air,  as  in  the  former  instance.  The  reac- 
tion is  expressed  by  the  equation 

HaSO4  +  FeS  =  FeSO4  +  H2S. 

Before  the  acid  is  put  in  the  generator  twice  its 
bulk  of  water  is  added. 

The  precipitation  of  gold  by  the  hydrogen  sulphide 
takes  place  according  to  the  following  reaction : 

2AuCl,  +  HaS  +  2H3O  =  2Au  +  6HC1  +  SO,. 


88  THE   CHLORINATION  PROCESS. 

The  solution  is  tested  in  about  one  hour  to  ascertain 
if  all  the  gold  is  precipitated  by  filtering  the  liquor  to 
be  tested  through  filter-paper,  and  adding  a  few  drops 
of  FeSO4.  When  no  precipitate  is  formed  on  the 
test  liquor  the  solution  is  allowed  to  stand  two  hours, 
and  then  drawn  off  to  within  4  inches  of  the  bottom 
of  the  tank,  and  passed  through  the  filter-press. 

The  precipitation  will  take  about  one  hour  and  the 
settling  two,  the  time  required  to  pass  through  the 
filter-press  about  three,  or  a  total  of  six  hours  to  pre- 
cipitate and  collect  the  gold.  When  this  is  compared 
with  three  days,  the  saving  in  time  is  noticeable. 
The  tank  of  7000  gallons  capacity  is  capable  of  hold- 
ing the  filterings  of  20  tons  ore  where  350  gallons  of 
water  is  added  in  leaching  and  washing. 

The  clean-up  occurs  once  or  twice  a  month,  when 
the  collected  filtrates  from  the  bottom  of  the  precipi- 
tating-tank  are  passed  into  the  pressure-tank,  and 
from  there  into  the  filter-press,  where  they  are  dried 
by  compressed  air.  These  filtrates  contain  impur- 
ities, which  will  require  careful  roasting  and  smelt- 
ing to  give  a  bullion  from  800  to  930  fine  gold.  Mr. 
J.  E.  Rothwell  has  precipitated  gold  from  solutions 
carrying  copper  by  this  method  without  precipitating 
the  copper,  which  is  almost  invariably  precipitated 
from  such  treatment  in  the  laboratory.* 

*  Potassium  nitrate  precipitates  metallic  gold  in  solution.     Potassa 


PRECIPITA  TION.  89 

Precipitation  becomes  difficult  when  impurities  are 
in  the  solution,  such  as  lime  and  magnesium,  which 
give  bulky  precipitates.*  This  can  be  obviated  by 
the  addition  of  acid,  or  by  filtering  through  charcoal. 
This  latter  process  is  carried  on  as  a  method  for  pre- 
venting the  bulky  precipitates  formed  by  ferrous 
sulphate  from  mixing  with  the  gold.  Just  what  the 
reaction  is  between  pulverized  charcoal  and  trichlo- 
ride of  gold  is  unintelligible,  but  the  charcoal  is 
certain  to  quickly  decompose  it,  and  collect  the  gold 
in  and  upon  its  surface.  If  the  lime,  magnesium,  or 
arsenic  be  fixed  or  held  in  solution  by  acid,  they  pass 
through  at  times  without  decomposition,  but  not 
always.  In  the  latter  case  the  utility  of  charcoal  as  a 
precipitant  over  ferrous  sulphate  is  nothing — in  fact, 
as  it  adds  new  complications,  is  a  hindrance,  and  to 
be  avoided.  There  may  be  more  or  less  slimes  as 
well  which  pass  the  filter :  these  collect  mechanically 
upon  the  charcoal,  and  when  burned  with  the  charcoal 
form  with  the  ash  a  mixture  which  requires  consider- 
able extra  work  to  get  rid  of;  then,  again,  at  times  the 
lime  and  magnesium  forming  a  precipitate  as  well  give 
virtually  a  new  gold  ore,  difficult  to  handle. 


or  soda  when  added  in  excess  will  leave  the  solution  clear,  but  on 
being  warmed  andtannic  acid  added,  will  separate  the  gold  (Johnson's 
Fresenius,  p.  191). 

*  £.  &>  M.  Journal,  vol.  LX.  p.  323. 


QO  THE   CHLORINA  TION  PROCESS. 

The  liquid  precipitants  have  all  the  same  disadvan- 
tage, viz.,  the  trouble  necessary  in  collecting  the  pre- 
cipitated metal ;  but  gold  does  not,  as  a  rule,  deliver 
itself  up  from  ores  as  bullion  without  trouble,  and 
that  is  to  be  expected.  Mr.  J.  T.  Blomfield  suggests 
the  following  plan  for  gold  precipitation : 

He  forms  subsulphide  of  copper  by  fusing  sulphur 
and  copper  together  (CuaS).  This  is  crushed  to 
pass  a  loo-mesh  sieve,  and  is  then  used  as  a  filter 
substance. 

The  liquor,  filtered  through  copper  sulphide,  showed 
no  gold  present  upon  testing  with  Fe,SO4. 

He  suggests  three  vessels  containing  this  precipi- 
tant :  the  first  to  catch  the  gold,  the  second  to  catch 
any  gold  which  may  have  passed  the  first,  and  the 
third  as  a  possibility. 

The  first  vessel,  Cu2S,  becomes  fully  charged  with 
gold  before  any  traces  appear  in  the  second.  This 
vessel  is  then  removed,  the  second  moved  to  take  its 
place,  and  the  third  the  second's,  while  a  new  vessel 
is  added  for  the  third. 

The  residue  is  fused  with  nitre  and  borax.  The 
reaction  he  gives  as  follows : 

3Cu2S  +  4AuCl3  =  Au3S3  +  Au  +  6CuCl2. 

This  equation  should  be  changed  somewhat,  as  the 
reaction  recorded  is  hardly  possible,  since  gold  will 


PRECIPITA  TION.  9 l 

not  unite  with  sulphur  in  the  cold,  and  under  such 
conditions  as  given.  The  following  is  therefore  ad- 
vanced as  the  probable  reaction : 

3CuaS  +  4AuCl,  =  4Au  +  6CuCla  +  38. 


CHAPTER    VIII. 
REFINING  PRECIPITATES. 

To  obtain  the  metal  precipitated  in  a  pure  form, 
that  it  may  be  handled  in  bulk  of  a  known  value,  it 
is  reduced  by  heat,  which  operation  is  termed  refin- 
ing. 

The  brown  oxide  of  gold  produced  by  the  action  of 
ferrous  sulphate  upon  the  trichloride  of  gold  is  col- 
lected, washed  on  filter-paper,  then  treated  with  acid 
to  remove  any  possible  impurities,  dried,  placed  in  a 
graphite  crucible  with  some  flux — either  soda  or 
borax,  and  then  placed  in  the  furnace  to  melt. 

Gold  bullion  is  said  to  be  fine  when  absolutely 
pure;  it  is  then  termed  1000  fine.  To  obtain  such 
bullion  requires  an  artist  and  very  fine  manipulation ; 
to  obtain  it  above  900  fine  is  not  as  difficult,  especially 
from  the  precipitates  of  ferrous  sulphate;  but  from 
the  precipitate  given  by  hydrogen  sulphide  the  task 
is  more  difficult  and  the  bullion  less  pure,  ranging  in 

fineness  from  800  to  950,  while  in  the  former  case  it 

92 


REFINING   PRECIPITATES.  93 

may  be  obtained  with  less  trouble  from  950  to  990 
fine. 

The  reddish-brown  precipitate  of  metallic  gold  ob- 
tained by  this  operation  is  washed  with  warm  water 
and  filtered  to  remove  any  acids  or  salts  of  iron  pres- 
ent, or  any  other  substances  soluble.  If  insoluble  salts 
of  metals  or  substances  are  present,  such  as  arsenic, 
zinc,  copper,  silver,  sulphur,  they  can  only  be  re- 
moved by  acids  to  reduce  them,  and  then  evaporation 
and  washing,  or  by  an  oxidizing  roast,  as  practised 
at  the  commencement,  but  on  a  smaller  scale,  in  a 
muffle. 

The  gold  oxide  precipitated  by  FeSO4  is  placed  in 
a  graphite  crucible,  and  with  a  little  borax  added  is 
introduced  into  the  furnace.  The  oxygen  is  driven 
off  by  the  heat,  the  gold  gradually  melting,  while  the 
borax  forms  a  liquid  slag  upon  the  surface  of  the 
molten  metal. 

This  slag  has  absorbed  the  impurities  which  were 
not  soluble  in  water,  and  as  the  slag  is  lighter  than 
gold,  by  carefully  skimming  the  surface  it  may  be 
nearly  all  removed. 

The  oxide  of  gold  is  a  peculiar  compound,  but  little 
known  or  understood.  Gold  when  heated  may  be 
deprived  of  oxygen,  but  it  will  not  unite  with  its  own 
or  other  oxides.  Gold  is  but  little  volatile,  and  may 
be  exposed  to  a  strong  heat  for  some  time  after  melt- 


94  THE    CHLORINATION  PROCESS. 

ing;  in  fact  it  is  frequently  boiled.  With  nitre  the 
boiling  is  so  marked  that  larger  crucibles  are  required 
as  well  as  covers  to  keep  it  within  bounds.  The 
opposite  is  the  case  when  volatile  metals  are  com- 
bined with  gold.  They  when  •  driven  off  by  heat 
carry  gold  with  them ;  noticeably  is  this  the  case  with 
the  metals  zinc,  lead,  arsenic,  antimony,  bismuth, 
and  tellurium.  To  obviate  this  as  much  as  possible 
the  gold  must  be  kept  below  a  boiling-point,  and  be 
protected  by  slag,  formed  from  a  suitable  flux.  Gold 
with  pure  borax  as  a  flux  assumes  a  whitish  color  in 
melting;  with  saltpeter  or  common  salt  it  retains  its 
rich  yellow  color. 

The  heat  should  at  first  be  mild,  but  may  be  grad- 
ually increased,  and  maintained  during  the  melting, 
and  for  some  little  time  afterwards.  The  crucible 
with  its  contents  is  then  removed,  and  the  gold  poured 
into  moulds  to  form  ingots  of  gold.  These  ingots 
are  sent  to  the  nearest  assay-office  (Government  assay- 
ofrice  wherever  possible),  tested  for  fineness,  and  sold. 

The  assay-offices  do  not  like  to  accept  base  bullion 
on  account  of  the  trouble  they  encounter  in  refining 
it,  nor  will  they  pay  as  good  price  for  impure  bullion 
on  account  of  the  extra  work  and  chances  for  loss  in 
further  refining. 

Mr.  Langguth  having  introduced  the  improved 
method  of  precipitating  referred  to  in  the  last  chapter, 


REFINING   PRECIPITATES.  95 

whereby  he  obtained  auric  sulphide,  was  obliged  to 
refine  these  sulphides  after  collecting  them  from  the 
filter-press. 

Hydrogen  sulphide  precipitates  gold  from  neutral 
or  acid  solutions.  From  the  cold  solution  the  pre- 
cipitate is  AuaSs ;  from  boiling  solutions,  Au2S.  These 
precipitates  are  insoluble  in  acids,  but  soluble  in 
"  aqua  regia" — a  mixture  of  nitric  and  muriatic  acids. 
If  we  should  use  the  latter  method  we  would  virtually 
have  the  same  experience  to  go  through  with  as  were 
we  to  use  FeSO4,  and  in  the  end  must  use  the  latter 
precipitant,  although  the  liquor  to  be  treated  would 
be  much  less  in  quantity. 

The  aurous  sulphides  may  be  dissolved  in  yellow 
ammonium  sulphide  or  by  yellow  potassium  or  so- 
dium sulphides,  but  there  is  nothing  to  be  gained  by 
so  doing. 

Mr.  Langguth  places  the  precipitates  and  the  filter- 
cloth  in  pans,  which  he  introduces  into  muffles  in  a 
roasting-furnace,  to  drive  off  the  sulphur,  arsenic,  and 
antimony,  or  whatever  other  volatile  substances  are 
present. 

This  oxidation  can  be  accomplished  in  about  three 
hours,  when  the  mass  presents  a  reddish  or  yellowish- 
brown  appearance.  If  salt  roasting  could  take  place 
without  loss  of  gold,  purer  bullion  could  be  obtained 
but  as  it  is,  care  has  to  be  used  in  first  heating  to 


g  THE   CHLORINATION  PROCESS. 

avoid  loss  of  gold  by  volatilization.  These  roasted 
sulphides  are  now  removed  from  the  muffle,  and 
being  hard  are  placed  in  a  pulverizing  drum  with 
cobble-stones  to  assist  pulverization,  after  which  the 
flux  is  added  in  suitable  proportions  and  thoroughly 
mixed  with  the  gold  sulphides. 

If  the  ore  treated  has  been  silicious  or  acid,  the 
flux  is  borax  or  soda;  on  the  other  hand,  if  it  has 
been  basic,  a  silicious  flux  of  sand  or  quartz  is  added. 
The  gold  mixture  is  now  placed  in  a  crucible,  and  this 
and  its  contents  placed  in  the  melting-furnace.  The 
sulphur  remaining  from  the  roast  is  partially  driven 
off  by  heat  and  absorbed  by  the  flux,  leaving  metallic 
gold  as  follows: 

Au3S2+  40  =  2Au  +  2SO,.* 

After  melting,  and  being  kept  at  a  high  tempera- 
ture some  time,  the  crucible  and  contents  are  removed 
from  the  furnace  and  poured  into  conical  moulds  of 
suitable  capacity.  The  bullion  separates  from  the 
slag  in  these  moulds  as  a  conical  button  in  the  bot- 
tom, from  which  it  is  taken  when  cool,  remelted,  and 
cast  into  ingots  for  shipment. 

The  slags  contain  considerable  gold,  and  are  there- 
fore pulverized  and  separated  by  water,  the  gold  being 

*  The  author  quotes  Mr.  Longguth's  equations. 


REFINING   PRECIPITATES.  97 

added  to  the  next  melting,  while  the  tailings  are 
mixed  with  lead  and  metallic  iron,  and  melted  in 
crucibles.  The  lead  bullion  resulting  is  cupelled, 
yielding  the  remainder  of  the  gold.  The  slags  from 
the  second  melting  are  too  poor  in  gold  to  handle. 

No  serious  losses  occur  by  this  method,  but  slight 
losses  do  occur,  and  from  what  has  been  said 
previous  are  due  to  vaporization  and  to  mechanical 
agencies. 


CHAPTER  IX. 
RESUME    OF   CHLORINATION. 

PLATTNER'S  application  of  chlorine  gas  to  cold 
gold  is  the  foundation  of  the  process.  Mr.  G.  F. 
Deitken  was  the  first  to  make  a  practical  and  commer- 
cial success  of  the  process  in  the  United  States,  at 
a  mill  situated  in  Grass  Valley,  California.  The  pio- 
neer works  of  the  East  were  under  the  charge  of  the 
present  editor  of  the  Engineering  and  Mining  Journal, 
Mr.  R.  P.  Rothwell;  they  were  situated  at  Deloro, 
Canada,  and  were  worked  by  the  Mears  process.  The 
pioneer  works  of  the  South  were  at  Haile  Gold  Mine, 
South  Carolina,  where  the  Theis  method  was  brought 
out.  The  pioneer  works  of  the  West,  as  far  as  barrel 
chlorination  is  concerned,  is  the  Golden  Reward  Chlo- 
rination  Mill,  Deadwood,  S.  D. 

The  tank-lixiviation  seems  to  have  made  way 
almost  entirely  for  the  barrel-chlorination  process,  and 
the  Mears  for  the  Theis. 

The  barrels  were  originally  constructed  to  contain 

one  ton,  but  are  now  made  to  hold  as  high  as  ten  tons, 

98 


RESUME   OF  CHLORINATION.  99 

the  latter  size  being  several  hundred  dollars  less  in  cost 
than  two  smaller  barrels  of  the  same  capacity.  They 
are  also  just  as  efficient  and  are  more  readily  operated, 
requiring  also  less  space  than  two  separate  barrels  of 
the  same  capacity. 

The  improvements  which  Hears  made  over  tank- 
chlorination  were  chiefly  those  which  economized  in 
time.  He  based  his  process  upon  pressure,  to  force 
the  chlorine  to  attack  the  gold  and,  further,  to  mix 
and  turn  up  the  ore ;  that  the  chlorine  gas  might  have 
more  opportunity  to  do  so,  he  rotated  the  barrel.  By 
these  improvements  he  reduced  the  time  from  three 
days  to  six  hours,  and  also  the  attendance  proportion- 
ally. Mr.  Theis  found  the  Mears  process  too  cum- 
bersome, and  introduced  the  chemicals  into  the  barrel, 
which  not  only  generated  the  gas,  but  produced  pres- 
sure sufficient  for  all  purposes.  This  was  a  most  im- 
portant improvement,  since  it  overcame  trouble  and 
expense  engendered  by  the  generation  of  gas  outside 
the  barrel.  It  also  overcame  several  mechanical  de- 
fects incident  to  such  generation,  as  well  as  doing 
away  with  the  pressure-gauge  and  its  leaky  goose-neck 
appendage. 

Mr.  Rothwell's  addition — the  filtering  from  the 
barrel  under  pressure — was  the  next  improvement; 
and  finally  may  be  added  Mr.  Langguth's  precipitation 
process, 


100  THE   CHLORINATION  PROCESS. 

These  improvements  have  increased  the  capacity 
of  the  plants  from  5  to  150  tons  daily  capacity,  and 
at  the  same  time  decreased  the  cost  of  operating,  and 
increased  the  extraction  in  a  given  time.  This  not 
only  speaks  well  for  the  process,  but  certifies  its 
usefulness.  The  original  Theis  plant  is  still  in  opera- 
tion, and  we  are  informed  has  paid  over  $5OO>OO°  m 
dividends  from  low-grade  sulphur  ores. 

The  mill  site  and  locality  will  determine  the  ar- 
rangement of  the  mill,  machinery,  and  labor-saving 
appliances  to  be  adopted. 

The  advantages  of  a  hillside  are  not  so  great  as 
would  warrant  the  building  of  a  long  tram-road  to 
obtain  a  gravity  fall,  as  in  gold  or  silver  milling, 
since  the  arrangement  must  be  as  automatic  as  possi- 
ble to  avoid  elevating  machinery.  But  in  our  case 
the  ore  will  have  to  be  conveyed  to  the  dryer  bin 
either  on  a  level  or  elevated,  and  from  the  dryer  to 
the  rolls  and  from  them  to  the  concentrators,  and 
finally  to  the  roasters,  cooling-floor,  and  leaching- 
barrels. 

The  preference  would  be  a  crusher  which  can 
handle  30  to  40  tons  per  hour,  this  being  passed 
over  |-inch  grizzlies  to  two  smaller  crushers  capable 
of  handling  the  product — say  100  tons  daily  for  the 
plant.  Of  the  100  tons  passed  to  the  large  crusher 
30  per  cent  will  be  of  a  suitable  size  for  the  dryer. 


RESUME  OP  CHLORINJA^JJI'ON: 


j    ibl 


This  relieves  the  small  crushers  of  30  tons  of  ore. 
The  small  crushers  will  produce  30  per  cent  of  ore 
which  will  screen  J  inch,  and  thus  relieve  the  dryer 
of  2 1  tons  daily,  for  the  moist  ore  will  generally  be 
found  in  the  first  crushings  and  screenings.  The 
small  ore  from  the  second  crushers  can  go  direct 
to  the  rolls;  the  finer  ore  to  the  dryer  will  also  be 
treated  quicker;  and  that  dried  ore  mixed  with  the 
fine  direct  from  the  crushers  will  screen  readily.  In 
some  cases  by  drying  ore  of  small  size  thoroughly  it 
has  been  found  that  ore  crushed  to  J-inch  mesh  will 
pass  the  rolls  and  screen  as  high  as  70  per  cent  through 
a  2O-mesh  screen.  The  advantage  of  dryers  and  the 
fine  crushing  thus  becomes  doubly  apparent.  The 
finer  the  ore  delivered  to  the  dryer  the  quicker  it  is 
deprived  of  moisture  and  the  better  the  product  of 
the  rolls. 

The  ore  which  passes  through  the  coarse  rolls 
should  screen  at  least  25  per  cent  3O-mesh,  25  per 
cent  -j^-mesh,  and  the  balance  between  £  and  J.  To 
throw  J-mesh  ore  on  a  No.  30  screen  is  very  destructive 
to  screens ;  for  this  reason  two  screens  are  used,  the 
inner  being  -J-mesh,  the  outer  3O-mesh.  The  product 
passing  the  -J-mesh  goes  to  the  fine  rolls,  the  balance 
back  to  the  coarse  rolls — possibly  25  per  cent.  The 
same  proportion  will  probably  hold  true  in  the  case 
of  the  fine  rolls,  25  per  cent  being  fine  enough  to  pass 


\bi  '¥&£  C'HLORINATION  PROCESS. 


the  3O-mesh  screen  with  one  pair  rolls,  while  50  per 
cent  would  pass  two  pairs  rolls  with   one  elevating. 

The  use  of  two  pairs  coarse  rolls  and  two  pairs  fine 
rolls  is  therefore  economy  in  power,  screens,  and  roll 
tires. 

If  concentrating  machinery  be  used  and  ore  crushed 
to  jo-mesh  separators  and  jigs,  fine  and  coarse  may 
answer;  but  as  there  is  much  ore  above  3O-mesh,  it 
may  be  necessary  to  use  separators  and  vanners  in 
preference  to  jigs  where  the  ore  crushes  easily. 

In  the  case  of  clayey  ores  rolls  will  not  answer,  and 
either  Chilian  mills,  Huntington  mills,  or  stamps 
must  be  used  for  crushing,  and  this  is  followed  by 
vanner  concentration. 

If  the  gold  be  coarse  and  can  be  saved  in  part  by 
amalgamation,  Huntington  mills  or  stamps  may  be 
used. 

If  fine  crushing  is  required,  Chilian  mills  or  stamps 
must  be  used,  the  limit  of  roll  crushing  being  30- 
mesh  screen. 

The  power  will  consist  of  a  first-class  automatic 
cut-off  engine  150  H.P.  being  sufficient  for  a  loo-ton 
mill.  This  should  have  feed-water  heater  in  prefer- 
ence to  condensing  apparatus,  unless  water  is  very 
valuable  and  needs  to  be  economized.  There  should 
be  an  electric-light  engine,  and  a  smaller  auxiliary 
engine  to  run  the  chlorinators  and  furnaces  at  night, 


RESUME   Of   CHLOR1NATION.  1 03 

in  case  all  the  power  were  not  needed.  This  latter, 
while  not  necessary,  will  be  found  to  be  economical  in 
fuel. 

The  boilers  and  feed-pump  are  a  part  of  the  plant. 
Flue-boilers  are  not  as  economical  as  tubular  boilers, 
although  the  latter  require  more  attention. 

The  dryer  is  a  cast  or  wrought  iron  revolving  cylin- 
der, similar  in  construction  to  furnaces,  with  cast-iron 
shelves  which  pick  up  the  ore  and  shower  it  through 
the  flames.  They  are  either  one  diameter  the  whole 
length,  with  jack-screws  to  elevate  one  end,  thus  giving 
a  quicker  or  slower  discharge  as  required ;  or  they  are 
made  larger  at  one  end  than  the  other,  to  work  the  ore 
forward  as  it  is  dried.  The  dryer  is  rotated  on  rollers, 
similar  to  revolving  roasting  furnaces. 

The  ore  is  fed  in  at  one  end  and  is  dropped  into  a 
pit  at  the  other,  from  which  it  is  conveyed  automati- 
cally by  elevators  or  scraper  lines  to  the  pulverizers. 
The  storage-tanks  and  other  appliances  of  the  plant 
have  already  been  mentioned. 

The  laboratory  is  a  necessary  appendage,  which 
should  be  fitted  up  with  assayer's  outfit,  also  a  roast- 
ing and  smelting  furnace,  and  in  case  of  amalgamation 
an  amalgam  retort. 

Pumps  also  may  be  required  about  the  plant,  and  a 
storage-room  for  chemicals  as  well. 

The   screens  are  either  hexagonal  or  round,  made 


104  THE    CHLORINAT10N  PROCESS. 

in  sections  that  wire  cloth  may  be  readily  replaced  in 
frames  which  fit  to  the  screen,  spiders,  and  frame. 
They  are  of  two  compartments,  as  mentioned,  the  in- 
side screen  being  heavy  wire  netting  or  metal  plate, 
with  holes  punched  in  it.  The  outer  screen  is  wire 
cloth ;  at  least  one  foot  should  be  allowed  between  tjie 
inner  and  outer  screens.*  A  Root  blower  will  prob- 
ably be  required  to  keep  down  the  dust  from  the  rolls 
and  screens. 

Salt-storage  bins  should  be  in  the  vicinity  of  the 
furnaces  where  chloridizing  roasting  is  practised,  and 
these  bins  should  be  of  sufficient  size  to  carry  a  good 
supply  unless  it  can  be  obtained  readily.  Automatic 
feeders  should  be  used  for  crushers  and  pulverizers ; 
also  automatic  machinery,  as  far  as  details  will  permit. 

*  The  Berthelet  separator  is  advanced  by  H.  F.  Brown  as  a  sub- 
stitute for  revolving  screens. 


CHAPTER  X. 
COST   OF   CHLORINATION. 

THE  factors  which  enter  into  the  cost  of  the  process 
preclude  giving  a  cost  which  will  cover  every  case. 

a.  These  are  cost  of  mining  and  transportation  to 
the  mill — two  items  which  will  vary  according  to  the 
character  of  the  rock,  the  depth  of  the  mine,  the  water 
encountered,  and  the  distance   of  the  mine  from  the 
mill.     This  cost  will  be  increased  or  decreased  by  the 
above,  and  further  influenced  by  the  situation  of  the 
mine  and  mill  with  reference  to  the  transportation  of 
supplies  and  product  from  railway  connections. 

b.  The  cost  of  milling  enters  into  the  calculation, 
which   depends  upon  the  character  of  the  rock,  the 
ease  with  which  it  is  crushed,  and  its  arrangements  in 
detail  for  close  and  automatic  work. 

c.  The    cost  of  concentration  and  the   amount   of 
concentrates  saved,  together  with  the  degree  of  care 
taken  to  obtain  clean  concentrates  for  roasting. 

d.  The  character  of  the  furnace,    and  the   degree 

of    desulphurization  dependent  upon  the  amount  of 

105 


106  THE   CHLOR1NAT10N  PROCESS. 

sulphur  in  the  ore,  or  other  substances  which  need  to 
be  eliminated,  determine  the  cost  in  this  department, 
which  must  necessarily  differ  in  different  localities  or 
for  different  ores  in  the  same  locality. 

e.  The  cost  of  chemicals  will  vary  according  to  the 
facilities  by  which  they  may  be  obtained  at  the  mill, 
and  will  also  vary  for  different  characters  of  ores,  thus 
making  their  cost  indefinite.      The  labor  required  in 
the  laboratory  for  precipitating  and  refining  is  also  an 
indefinite  consideration. 

f.  The  superintendent's  and  office  expenses  will  vary 
for  each  mine,   a  good  manager  generally,   however, 
saves  his  wages  several  times  over  during  the  year. 

Much  is  said  about  this  process  for  low-grade  ores; 
and  for  the  benefit  of  those  contemplating  the  use  of 
this  process  on  low-grade  propositions  we  would  sug- 
gest they  advise  with  a  mining  engineer  before  so 
doing,  and  not  to  attempt  it  on  ores  carrying  less 
than  $6  per  ton  of  gold  if  the  mine  be  not  well 
developed,  in  which  latter  case  with  favorable  circum- 
stances the  process  should  be  remunerative. 

Mines  which  are  but  partially  developed  do  not 
offer  enough  stoping  ground  for  steady  supply  of 
suitable  ore,  and  development  is  always  more  expen- 
sive than  mining.  With  mines  well  opened  on  plenty 
of  stopes  the  mining  expenses  are  reduced,  and  more 
than  cover  the  cost  of  development.  The  average 


COST  OF  CHLORINATION.  IO? 

cost  of  mining  will  be  about  $1.45  per  ton  where 
power  drilling,  hoisting,  and  pumping  is  carried  on, 
which  figure  also  includes  timbering. 

The  dumping  and  tramming  to  the  mill,  and  the 
dumping  there,  will  add  ten  cents  per  ton  to  the  above 
cost. 

The  milling  will  average  on  hard  rock  where  stamps 
are  used  $i,  and  the  concentrating  and  the  handling 
of  tailings  and  product  about  50  cents  more. 

After  concentration  the  cost  of  roasting  may  be 
considered,  and  this  will  vary  from  30  cents  in  the 
Cripple  Creek  district  to  $3.75  in  others.  At  the 
Haile  Mine,  where  wood  and  labor  are  cheap  in  com- 
parison with  other  localities,  the  cost  is  placed  at 
$2.62,  for  a  ton  of  roasted  concentrates,  oxidized  to  a 
"dead  roast  "  so  that  but  0.25  per  cent  of  sulphur 
remains  in  the  ore. 

Then  in  order  comes  the  cost  of  chemicals,  which 
will  vary  from  $2  to  $3  per  ton  of  concentrates,  while 
the  labor  and  other  expenses  will  add  about  $5  to  it. 

In  taking  these  different  items  up  in  detail,  it  must 
be  borne  in  mind  that  we  are  dealing  with  concentrates, 
and,  further,  roasted  concentrates. 

If  we  have  an  ore  running  $6  per  ton  as  it  comes 
from  the  mine,  and  we  can  concentrate  90  per  cent  of 
the  value,  the  ore  should  assay  $5.40.  If  the  ore  runs 
33i  per  cent  in  sulphur,  one  ton  of  roasted  ore  will 


108  THE  CHLORINATION  PROCESS. 

require  1.3  tons  of  concentrates — possibly  more;  or  13 
tons  of  raw  ore  will  make  I  ton  of  roasted  concen- 
trates. The  value  of  the  roasted  ore  will  also  be 
raised,  by  its  quantity  being  lowered,  provided  no  loss 
occurs  other  than  mentioned,  the  value  of  13  tons 
mine  ore;  \\  tons  of  raw  concentrates  or  I  ton  of 
roasted  ore,  will  be  $70.20. 

With  miners'  wages  at  $2.50  and  coal  at  $5,  if  2.5 
tons  of  ore  are  drilled,  shot,  cobbed,  loaded,  and 
hoisted  per  man  employed  at  the  mines  the  work  is 
excellent. 

The  cost  then  would  be  about  as  follows: 

Labor $ i  .00  per  ton. 

Powder,  fuse,  and  caps 15    "      " 

Fuel ., 15    "      " 

Timbering 15    "      " 

Supplies 10    "      " 

$1-55 

Or  $19.15  for  one  ton  of  concentrates. 

Adding  10  cents  per  ton  of  mine  ore  for  transporta- 
tion, which  includes  wear  and  tear  on  rolling-stock  and 
motive  power,  oil-waste,  runners  and  dumpman's 
wages  at  the  mill,  the  cost  for  13  tons  is  $1.30. 

With  wood  at  $2.50  per  cord  and  labor  $2.00,  the 
cost  of  roasting  one  ton  of  concentrates  would  be — 


COST  OF  CHLORINATION.  1 09 

i  cord  wood  at  $2.50 $1.25 

12  hours'  labor  at  20  cents 2.40 

Motive  power 35 

Cost  of  roasting  1.3  tons  concentrates $4.00 

The  cost  of  milling  would  be,  with  60  stamps,  in 
hard  rock: 

Labor $0.35 

Supplies 02 

Fuel 25 

Machinery » . . .        .10 

Oil  and  waste 03 

Illumination .02 

Lumber , 02 


$0.79 

Or  for  13  tons  $10.27. 

The  cost  of  concentration  would  be  about  as  fol- 
lows: 

Labor $0.26 

Repairs o.  10 

Supplies o.  10 

Fuel..  0.15 


per  ton,  or  $7.93  for  13  tons  ore. 


110  THE   CHLORINATION  PROCESS. 

The  chemicals  may  be  more  or  less  than  the  quanti- 
ties given  ;   we  have  endeavored  to  take  an  average : 

30  Ibs.  chloride  of  lime  at  3  cents .90 

35    "     sulphuric  acid  (H2SO4)  at  3  cents 1.05 

10    "     HaSO4  for  settling-tanks .30 

10    "     H2SO4  for  ferrous  sulphate .30 

40    "    salt  at  £  cent .20 

Labor  of  2  men  at  $2 4.00 

Labor  of  I  man  at  $3.50 3.50 

Motive  power .35 

Laboratory  expenses. 1.50 


Cost  of  chlorinating $12.10 

We  have  not  added  the  proportional  part  of  super- 
intendent's and  office  expenses,  which  would  reach  10 
cents  per  ton  raw  ore,  or  $1.30  per  ton  of  concentrated 
roasted  ore.  There  is  still  another  item,  such  as 
amortization,  which  may  be  placed  at  25  cents  per  ton 
of  concentrates  roasted.  Our  cost  is  then  as  follows : 

Mining  1 3  tons  ore $19.15 

Transportation  to  mill,  etc 1.30 

Milling 10.27 

Concentrating 7.93 

Roasting 4.00 

Carried  forward 42.65 


COST  OF  CHLORINATION.  Ill 

Brought  forward $42.65 

Chlorinating  and  recovery 12. 10 

Superintendent  and  office  expenses..  1.30 

Amortization 3.25 

Total  cost  of  chlorinating  one  ton 

roasted  ore $59-3° 

Or  per  ton  of  mine  ore  $4.56. 

The  total  value  of  the  ore  concentrated  is  $70.20; 
as  we  cannot  extract  all  of  this  value,  only  about 
Q2-f-  per  cent,  we  have  for  a  margin  of  profit  the  dif- 
ference between  $64.58  and  $59.30,  or  $5.28 — about 
40  cents  per  ton  of  ore  mined. 

This  margin  we  consider  too  small  for  mining  and 
milling  for  safe  work,  as  it  does  not  allow  10  per  cent 
for  contingences  and  requires  that  100  tons  be  mined 
daily  for  155  consecutive  days  before  the  interest  on 
the  investment, if  it  be  $100,000,  is  paid.  With  a  free- 
milling  gold  ore  this  profit  could  be  increased  from 
$52.80  on  a  loo-ton  daily  mill  to  $116,  and  only  ex- 
tract 60  per  cent  of  the  gold ;  the  remainder  in  the 
tailings  could  then  undoubtedly  be  concentrated  and 
treated  at  a  profit  sufficient  to  warrant  the  expendi- 
ture of  the  money  on  the  additional  plant. 

The  first  cost  of  installation  is  high,  because  of  the 
necessary  crushing,  concentrating,  roasting,  filtering. 


112  THE   CHLORINATION  PROCESS. 

and  precipitation  requirements.  A  complete  concen- 
trating plant  for  50  tons  daily,  which  includes  crush- 
ing plant,  will  cost,  according  to  its  elaborateness, 
between  $30,000  and  $40,000. 

For  chlorination  must  be  added  the  furnaces  and 
other  paraphernalia  equally  expensive,  which  with 
buildings  will  bring  the  cost  up  from  $60,000  to 
$80,000.  The  process  is  not  able  to  recover  coarse  flake 
gold,  and  does  not  recover  silver,  but  under  certain 
conditions  it  is  the  best  process  known.  It  has  in  its 
favor  the  treatment  of  high-grade  sulphurets  with  a 
high  extraction.  In  this  respect  it  is  superior  to 
cyanide,  even  when  the  ore  is  roasted  for  both  proc- 
esses. It  covers  a  field  distinctly  its  own,  and  has 
certainly  come  to  remain  as  one  of  the  metallurgical 
operations  of  the  future. 

In  some  instances  tailings  can  be  treated  at  a  very 
low  figure ;  in  others  the  cost  of  roasting  varies  from 
30  cents  to  $4.60  per  ton,  and  the  chlorinating  from 
$2.18  to  $4.60  per  ton  of  concentrates.  The  Golden 
Reward  has  reduced  the  cost  to  $3,  while  the  Haile 
Mine  cost  has  been  reduced  from  $4.84  to  $3.50  per 
ton.  We  may  expect,  therefore,  greater  reductions 
when  chemicals  and  roasting  are  reduced  in  cost. 

For  the  benefit  of  those  who  inquire  why  this  proc- 
ess has  not  been  more  generally  adopted,  it  should  be 
stated  that  those  who  run  mines  are  not  capable  of 


COST  OF  CHLORINATION.  113 

carrying  it  on,  and,  knowing  their  weakness,  let  it 
alone ;  again,  parties  having  mining  machinery  to  sell 
advise  the  use  of  their  machinery  rather  than  chlorin- 
ation ;  and,  lastly,  mine  owners,  thinking  they  will 
have  to  pay  higher  salaries  to  good  men,  are  willing 
to  suffer  loss  of  gold  rather  than  do  so.  Other 
matters,  such  as  inability  to  purchase  the  necessary 
plant,  must  be  considered  as  a  further  hindrance  to  its 
introduction.  For  that  reason  custom  mills  do  a  fair 
business  in  some  localities. 

Wherever  chlorination  has  been  introduced  it  re- 
mains, thus  testifying  to  its  worth  as  a  means  for 
extracting  gold  from  refractory  ores. 


FINIS. 


INFORMATION. 


WATER   FOR   MILLING   PURPOSES. 

For  stamps,  1.2  to  3  gallons  per  stamp  per  minute. 
"     plain-belt  vanners,  f  to  2  gallons  per  minute. 
"     corrugated-belt  vanners,  i£  to  3  gallons  per  minute. 
NOTE. — To  the  vanner  water,   which  is  clear,  must  be 
added  the  pulp  from  the  stamps. 

For  boilers,  8  gallons  per  horse-power  per  hour, 
each  settler,  i  gallon  per  minute. 

"      pan,  2  gallons  per  minute. 

chlorinating  barrel,  80  to  100  gallons  per  ton  of  ore. 
Wash-water,  200  to  300  gallons  per  ton  of  ore. 

Of  the  above  water,  50  per  cent  of  that  used  for  engine 
can  be  condensed  and  used  over  again. 

Of  that  water  used  by  stamps,  settlers,  and  pans,  all  but 
20  per  cent  may  be  settled  and  used  over  again. 

The  chlorinating  and  wash  water  cannot  be  used  again 
in  the  leaching  or  washing  process  where  ferrous  sul- 
phate or  hydrogen  sulphide  is  the  precipitating  agent,  but 
it  may  be  used  when  charcoal  is  the  agent. 


METRIC   CONVERSION   TABLE. 

7000  grains  =  avoirdupois  pound. 

5760  =  troy  pound. 

Av.  Ibs.  X  1.21527  =  troy  pounds. 
"    ozs.  X  .9115      =     "     ounces. 

One  gramme  =  15.433  grains. 

One  kilogramme      =  2.2047  pounds  av. 
One  =  2.6778       "       troy. 

114 


IN  FORM  A  TION.  1 1 5 

Meter  =  39-3710  inches. 
Millimeter  =      .03937    " 

i  inch  =  .0254  meters, 

ft.           =       12  inches  —  .3048 

yd.         -      36      "  =  .9144 

fathom  =        6  ft.  1.8287 

pole       =      i6-j-"  =  5^0291 

furlong  —    660   '  =  201.16 

mile       =  5280    "  =  1609.351 

To  change  millimeters  into  decimals  of  an  inch,  multiply 
the  number  of  meters  by  .03937. 

^j  of  an  inch  is,  in  decimals,  .015625 

A  "    "      "     "    "         "          -°3I25 
A  "    "      "      "     "         "          .0625 

£    "    "      "     "     "         "          .125 
i    «    a      a     «     «         «  g 

1      it     «          «         u       «  «  ^Q 


THE  THERMOMETER  RULES  FQR  CONVERT- 
ING FAHRENHEIT  INTO  CENTIGRADE  OR 
REAUMUR. 

Water  boils.  .Fahr.  212°,     Cent.  100°,     Reau.  80°.    • 
Ice  melts....    "         32°,  o°,  o°. 

2i2°-32=i8o  and  ^=^=1.8  C,  or  W  =1=2-25  R^au. 

Temp.  Fahr.  —  (-§•  -f-  32)  Cent,  or  (}  -f  32)  R^au. 
"      Cent.  =  (|  —  32)  Fahr.  or  f  Re"au. 
"      Reau.=  (|  —  32)  Fahr.  or  f  Cent. 

To  convert  Fahr.  to  Cent.:  Subtract  32  and  divide  by  1.8. 
To  convert  Fahr.  to  Reaumur:  Subtract  32  and  divide  by  2.25. 
To  convert  Cent,  to  Fahr.:  Multiply  by  1.8  and  add  32  to 

the  product. 

To  convert  Cent,  to  Reaumur  :  Deduct  -£. 
To  convert  Reaumur  to  Fahr.:  Multiply  by  2.25  and  add  32 

to  the  product. 
To  convert  Reaumur  to  Cent.:  Add    . 


Il6  THE   CHLORINATION  PROCESS. 

SCREENS. 

Wire  screens  take  their  numbers  from  the  meshes  in  the 
linear  inch;  slotted  screens,  from  the  number  of  needle 
used  in  punching  the  plates.  The  slots  are  usually  -J  inch 
long,  running  diagonally,  but  can  be  obtained  either  run- 
ning lengthwise  or  crosswise  of  the  sheet. 

Wire  screens  pass  a  greater  product  than  slot-screens, 
but  wear  faster. 

Number  Meshes  Width  Equivalent 

of  Needle.  per  Inch.  ?f  S,lot«  Birmingham 

Inches.  Gauge. 

1  12  .058  22$ 

2  14  .049  22$ 

3  16  .042  22$ 

4  18  .035  22$ 

5  20  .029  23^ 

6  25  .027  24 

7  3°  -024  24^ 

8  35  .022  25 

9  40  .020  26 
10  50  .018  27 
n  55  .016  28 
i2  60  .015  28 

Higher  numbers  of  wire  screens  can  be  obtained  of  the 
dealers. 


COLORS  EXPRESSIVE  OF  TEMPERATURES  COR- 
RESPONDING TO  DEGREES  FAHRENHEIT 
AND  CENTIGRADE. 

Faint  red 960°  F.  515°  C. 

Dull     "    1260°  F.  680°  C. 

Cherry-red 1650°  F.  898°  C. 

Yellow  or  orange  .  2010°  F.  1099°  C. 

White 2370°  F.  1299°  C. 


INFORM  A  T10N. 


117 


The  following  metals  volatilize  below  red  heat :  Arsenic, 
at  356°  F.  or  180°  C.;  mercury,  slightly  at  ordinary  tem- 
perature, boiling  at  680°  F.  or  362°  C. 

The  following  metals  volatilize  at  red  heat :  Potassium, 
sodium,  tellurium,  magnesium,  zinc. 

The  following  metals  volatilize  at  white  heat :  Zinc,  anti- 
mony, lead,  bismuth. 

Metals  which  give  up  their  oxygen  to  heat :  Mercury, 
silver,  gold,  platinum,  palladium,  rhodium,  iridium,  and 
osmium. 

Metals  which  retain  their  oxygen  at  high  temperatures, 
and  cannot  be  reduced  by  heat  alone  :  Potassium,  sodium, 
calcium,  magnesium. 


CONCERNING   METALS. 


Name. 

Specific 
Gravity. 

Chemical 
Symbol. 

Atomic 
Weight. 

Fusibility, 
Fahr. 

2.6 

Al 

27.4 

very  high  h. 

6.7 

Sb 

122 

032° 

Arsenic           .  .  . 

c   7 

As 

7c 

4OO 

9.74 

Bi 

2IO 

476° 

Copper  

8.8 
7.6 

Cu 
Fe 

63.4 
56 

1996° 
2500° 

Lead   

114. 

Pb 

2O7 

CQ«° 

iq    eg 

Hg 

2OO 

—  3Q 

Zinc  

6.8 

Zn 

65 

77O° 

Tellurium  
Silver  

6.1 
10.  ^ 

Te 

Ag 

128 
I  O8 

600° 

TQCO° 

Gold   

IQ.4 

Au 

107 

2OOO° 

5.85 

Mn 

55 

very  high  h. 

Potassium  
Sodium  

.865 
.072 

K 

Na 

39-i 

21 

136° 

IQ4° 

Platinum 

21    2 

Pt 

IQ7    4. 

very  high  h 

IX.  3 

Pd 

106.6 

high  heat 

Nickel  

8.2 

Ni 

R8.8 

«       « 

5.Q 

Cr 

52.2 

n         a 

INDEX. 


PAGE 

Aaron,  C.  H..    28 

Ammonia  test  for  Cl 63,  73 

Aqua  regia 5 

Arsenic 23 

Assays '. 12 

"      offices * 95 

Auric  chloride 3,  60 

' '      oxide 93 

"      sulphide 90 

Austin  smelting  process 33 

Barrel  charging 71 

"      chlorination 66 

"      construction 69 

"      pressure  in 81 

Bismuth 23 

Bleaching-powder 4,  61 

Bromide 2 

Bruckner's  cylinders 48 

Bullion , 83,  94 

Butters,  C 28 

Calcium  carbonate 65 

"        chloride 5,65 

"        hydrate 4 

"        sulphide 64 

Charcoal  reaction " 89 

Charging  the  barrels 72 

ChloridG  of  calcium 5,  65 

"       "  gold 3 

"        "  silver 3,  23 

"  sodium 7 

119 


120  INDEX. 


Chloridizing  roasting 3,  7,  22 

Chlorine 3,  72 

"        tests  for   63,77 

Chlorination I,  17 

"          advantages  of 1 1 1 

barrel 66 

"          cost  of , 112 

"          plant  for 62,111 

"          ore  for II,  20 

"          recovery  by 64 

"  roasting  for 22 

Chloro-aurates 60 

Christy,  Prof 29 

Cobalt 24 

Collection  of  precipitates 83 

Concentration , 8,  16,  17,  102 

"  cost  of 120 

Construction  of  barrels 68 

"  fibers 75,  79 

"  "  tanks 62 

Copper  sulphides 23,  90 

Cost  of  chlorinating 18,  112 

"     "chemicals no 

"    "concentrating 109 

' «    "  milling. 109 

"    "mining 106 

"    "plant in 

"     "roasting 112 

Crushing  ores 12 

Cyanide .......   2,112 

Dead  roasting 22,  32,  37 

Dead  roast,  test  for 34 

Decanting  the  liquor 77 

Decomposition  of  sulphates 35 

Dryers 56,  100,  102 

Experiments  by  Aarons,  C.  H   28 

"  "    Butters,  C 28 

"  "    Christy,  Prof 27 


INDEX.  121 

PAGE 

Experiments  by  Plattner 27 

"   Stetefeldt,  C.  A 28 

Ferrous  sulphate 76,  82 

*'  "       manufacture  of. 83,92 

Filtering 25,  75,  89 

"        precipitates 85 

"        pressure  in 75,78 

"        time  of 77 

"        with  sand 75 

Fine  ore 24 

Fluxes 83,  90 

Furnaces,  roasting 37 

Fusion 21 

Galena 23 

Gold  chlorides 3,60,76 

"     fineness 83,  88,  92 

"     loss  of 26,  32 

"     oxide 82,  93 

"     precipitation 87 

"     refining 92 

"     recovery 64 

"     relative  solubility  in  Br,  Cl,  Cy 2 

"     sulphide 84 

' '     soluble  in 2,  5 

"     test  for ' 83,88 

"    volatility  of 93 

Hydrochlo-auric  acid 6 

Hydrochloric  acid 4 

Hydrogen  sulphide 87 

Hovvell  furnace 53 

Iodine  test  for  Cl 85 

Iron  sulphides 22 

Laboratory  expenses no 

Langguth's  methods , 84 

Leaching 25.  60 


122  INDEX. 

PAGE 

Leaching,  partial   73 

"          time  of 63,  72 

Lime,  slacked 4 

"      precipitated 64 

Lixiviation I,  10 

Loss  by  roasting 33 

"    of  gold,  see  Gold. 

"     "  silver,  see  Silver. 
Low-grade  ores 10 

Mears  process ....   65 

Mechanical  roasting 37,  57 

Melting 93 

Metal  mining 9 

"  "      cost  of 106 

"     salts ,....  94 

Mercury 23 

Milling,  cost  of 7 

Mining  advice 10,  106 

Muffle-roasting 29 

Muriatic  acid 4 

Newberry-Vautin 67 

Nickel 23 

Nitrate  of  silver  test 85 

Ores,  drying 51,  63 

"      fineness  of 24 

"      leaching , . .  .  31 

"      loss  in  weight  of 34 

"     low-grade. 10,  105 

"      of  base  metals 25 

"     preparation  of 11,22 

"     refractory 16,  22 

"      roasting  of 20,33 

"      treatment  of 8 

"      sulphide 7,16,23 

Oxland  furnace,  see  Furnaces. 

Oxidation 3,  7,  20 

Oxides  of  gold 82,  93 

Oxidizing  roasting 40 


INDEX.  123 

PAGE 

Process,  chlorination,  see  Chlorination. 

"       choice  of I,  18 

"       cyanide ».... 1,18,112 

"       Mears' 65 

"       Newberry-Vautin 67 

"       Planner's 6,  61 

"       Russell 1,23 

"      Theis' 65 

Phosphorus 24 

Pile-roasting 3^ 

Precipitates,  collection  of 83 

"  Langguth's 83 

"          time  for 81,89 

"          treatment  of 83,93 

Precipitation. ...    81 

by  FeSO4 82 

by  H2S 65,  84,  87 

difficulties 88 

"  fractional • 88 

of  gold 81 

Potassium  nitrate 93 

"          sulphide 96 

Pressure  in  filtering 75 

"  process 67,  75 

Pulverizing  sulphides • 9°>  9° 

Rabbles 24 

Recovery  of  gold 73,  75 

Refining 94 

Refractory  ores,  see  Ores. 

Reverberatory  furnace 38 

Roasting,  see  Furnaces. 

'•        cost  of no 

dead 22,32 

"         furnaces 3° 

"         in  piles 37 

muffles.... 27 

"         precipitates » 93 

"         ores 22 

1 '        temperature  in -. "  •  •  3° 


124  INDEX. 

PAGE 

Roasting,  temperature  test 34 

"  theory  of 35 

"  time  of 32 

Russell  process I,  23 

Salt 7,  26,  34 

Sand  filters 75 

Settling  tanks 64 

Silver  chloride , 3,  20,  23 

lossof 22 

S'ags 93 

Soda  and  tannic  acid. 91 

Sodium  chloride 3,  21,  26, 

"       sulphide 96 

Storage  tanks. 81 

Sulphates 33 

Sulphides 16,  23,  64 

"        and  heat 36 

Sulphuric  acid  a  fix 81 

"  "    for  chlorine 4,  16 

"      "  FeSO4 76,82 

"      "  HaS 88 

Sulphurous  acid  generator 85 

Sulphuretted  hydrogen 87 

Tellurides 23,  28 

Temperature  in  roasting 30 

Test  for  chlorine 63,  73,  76 

"     "    gas ...  68 

"     "    gold 74,83,88 

"     "    roasting 34 

"     "    wash-water 76 

Theis  process 65 

Time  in  filtering. 77,  79 

"      "  leaching 63,  72 

"      "  precipitating 85,89 

"      "  roasting 32 

Titanium 39,  100 

Treatment  of  precipitates  83,  93 

"  ores..  .  8 


INDEX.  125 

PAGE 

Treatment  of  slags 93 

Trichloride  of  gold 4»  7° 

Vein  rock I2 

Wash-water.. 64,77 


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